bearing clearances

Discussion in 'Rotating Assemblies' started by grumpyvette, Jan 12, 2010.

  1. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    Understanding Today’s Bearing Clearance Recipe

    by Engine Builder Staff - Feb 6, 2015
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    [​IMG]What’s needed to keep the rotating assembly rotating? It’s a recipe that includes three key ingredients: the bearing, the crankshaft and the oil. But understanding the part each plays as part of this recipe is what’s needed to keep your engines’ bearings from failing.

    When it comes to understanding what keeps a bearing from failing you must keep in mind that each ingredient, the bearing, the oil and the crankshaft all play a part in the recipe for long bearing life.

    Changes in engine oil have caused manufacturers to make changes to bearings and crankshafts to make the recipe provide better bearing life than we have ever seen.

    Let’s get a picture of how little clearance can be between a bearing and the crankshaft. The large circle represents what is supposed to be the diameter of a human hair. Note how small the .0001” circle is compared to a human hair.

    Now let’s take a good look at the clearances between a crankshaft and bearings. Recently there has been a good awareness about the “hydrodynamic wedge” of oil that moves the crankshaft off center when the engine is running. That means that a bearing clearance that might be .002” could only be .0002” to .0003” between the bearing and crankshaft when the hydrodynamic wedge of oil moves the crankshaft off center. This is OK as long as this slight film of oil between the crank and bearing is not ruptured allowing crankshaft to bearing contact.

    The quality of OE crankshaft surfaces has kept pace with the changes in oil. OE’s are requiring surface finish requirements that were almost pure fantasy years ago. Keep in mind that not only the surface finish is critical to keep from rupturing the oil film but the geometry of the journal is as critical. Out of roundness and taper used to have wide tolerances. It used to be common to allow a combination of .0005” out of round and taper. This tolerance did not create problems with heavier oil and larger bearing clearances, but today it is not acceptable.

    High quality crankshaft manufacturing machines and the use of hard shoe polishers have changed the way OE’s and aftermarket suppliers can mass produce crankshafts that have the right ingredients to make the oil, bearing and crankshaft recipe work.

    If your shop is lucky enough to have a profilometer you should be able to measure the surface finish of a journal. With a good quality micrometer and an experienced machinist the size, roundness and flatness can be measured. The problem some shops have is that their measuring equipment can also be experienced and might not be as accurate as it was when it was new.

    Just be careful if you are using a belt style of crankshaft polisher that you don’t change the geometry. Flatness and taper are critical to keep from rupturing the very thin oil film between the crank and bearing. You can’t have good bearing life if the crankshaft is not good enough to be part of the recipe.

    By Lyle Haley, The Shop Doc
  2. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

  3. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    Hi Grumpy,

    I caught your article on Journal measurements and would really appreciate your perspective on the followng questions. (background blather appears below the questions)

    1) Shouldn't the journal in your article have looked shinier if it was a finish cut ? (see background below)

    90% or more of the pictures were and are posted by other people on the internet , and merely used to illustrate, yes ideally a finish journal should be as close to mirror finished and glass smooth as you can get it, but level, from end to end except for the curved machined bevel,on each end ,its diameter concentric and even in its dimensions its intended and ideally standard size or standard under size, is critical

    2) Is there a need to plasti-guage a daily driver crank (no racing) that's been resurfaced to a matching set of bearings ? I actually thought about plasti-gaging the journals rods from underneath (one location) but taking 6 measurements per makes removing the crank seem easier to do. I will definitely check for debris and burrs on future kits however.

    its always a good idea to verify machine work, theres always some machine shops that either do low quality or shoddy work, and you can,t expect an engine to operate correctly if the clearances or consistency of the bearing and lubrication surfaces are not correctly set, you would not bet the first or last guy to be told a crank was cut to a size that was not the true size or sold the wrong bearings for an application, you should always verify machine work, with both a precision measuring tool and plasti-gauge
    keep in mind fluids like oil, are not compressible the resistance to flow increases as the pump is turned simply because the oil flow exiting the oil pump is flowing through the bearing clearances thus the oil flow exiting the oil pump is being restricted, if the engine was rotating the restriction too oil flow is reduced.
    if the oil pumps pressure relief valve opens as designed max resistance is limited to the pump producing flow rates that will build until resistance to that flow volume reaches about 60 psi, when the pressure relief valve opens re-ce-recirculating additional oil flow back to the low pressure side of the oil pump.
    the first few rule's of GRUMPY'S engine assembly





    (2) if in doubt, about how to do anything, on an engine, do some detailed research,
    find and compare at least 3-5 valid trust worthy sources info,
    read the instructions over again, several time's very carefully
    and if available watch several related videos.

    (3) if any component will not easily function as designed or requires a good bit of physical force to install ,
    or your not 100% sure your doing something CORRECTLY

    theres a reason, and you better verify your clearances are correct , and your following the instructions before you proceed.

    (4) never assume the parts you purchased can be used without carefully , cleaning them prior too,
    checking the physical condition, verifying clearances and using the correct sealant, lubricants etc.

    (5) the quality of a component is generally at least loosely related to the cost to produce it,
    and the amount of detailed research and quality machine work that went into its production.
    if you got a significant reduced price, theres typically a reason.
    it might simply be because a new improved part superseded the one you purchased,
    but it might be a far lower quality imported clone with lower quality materials and machine work.
    its the purchasers responsibility to research quality.

    (6) if you did not do the work personally or at least take the effort to verify it was done correctly and personally verify clearances

    ITS almost a sure thing that it was NOT done , correctly, and yes that mandates you fully understand what your looking at,
    and how the components are supposed to function and have high quality precision measuring tools.



    3) Do you have any experience doing the crank kit right in the Caprice (without removing engine)

    no I have always found it best too assemble an engine after placing it on a decent engine stand
    if your going to assemble an engine you should buy or rent a decent engine stand, and a few basic tools

    4) I didn't understand from the graphs posted. What is a 180 'bearing' whatever vs a 270 ? and which is best and why ?
    a 180 degree bearing has only the upper in the block grooved to improve oil flow,a 270 degree has the oil feed groove extend further 45 degrees on each lower bearing shell




    MAIN BEARINGS WITH 360 degree oil grooves

    I've got my grandmothers 86 caprice that still looks really good and never fails to turn heads. Bone stock, but most of the cars been painted over 23 years I've owned it by a company that guarantees paint for life. More importantly this car has been just amazingly reliable over the years. Now that my son is ready to drive it (17) I'm noticing the engine shudders as second winds out so the bottoms loose with mileage just under 199K. I'm far from expert so did some googling and found your journal article.

    I have done two crank shaft kits over the years - while the engines remained in the vehicles. 1975 and 1979 350 Chevy pickups. After reading the article I"ll have to say I don't recall checking nor deburing the oil passages - which would have been colossally easy to do. Will do so going forward. In both instances I pulled the crank had them resurfaced then bought bearings of matching thickness. The shop I turned the crank in 1990 has since closed so I'm checking out machine shops.

    Note: The journal image in the article didn't look very shiny. Back during my 1990 crank kit install, I had the good fortune of comparing the journals of a crank that had been rough cut (per a racing engine builder) with my crank done by a shop owned by guys that raced engines. The difference was remarkable. I could easily see my pores, individual pores etc.. in the journals. They were mirrors. I installed it and drove my pickup daily to work 15 more years until gas hit $4/gallon then switched to the subject Caprice. Now my
    Last edited: Jul 26, 2018
  4. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    paint, marker etc. tends to wash off, you really should lightly die stamp the main caps

    on paired connecting rods, bevel side faces out toward crank counter weight, smooth side faces the adjacent rod face




    Clearing the Air on Bearing Clearances



    Engine connecting rod and main bearing clearances have tightened in recent years for many racing applications, but there are caveats to this trend that engine builders need to acknowledge before bending traditional guidelines, especially when adapting those practices to street engines.

    “It’s easy to whip these numbers out. But you have to remember, how good is the shaft?–Bill McKnight, Mahle Aftermarket

    “There are specific classes that really go towards the tighter oil clearance and thinner oil,” says John Himley, engine builder at CNC Motorsports. “NASCAR and obviously Pro Stock, those guys nit-pick every tiny bit of horsepower they can out of that engine. That’s where you see the tighter oil clearances being used.”

    As with any high-end racing innovation, there’s always the temptation to leverage that technology into sportsman classes and even to street vehicles. Remember the 55-degree valve angle story? There are comparable cautions to consider when setting bearing clearances.

    Plastigage, which has been available since the ’40s and hasn’t changed much since, may work for a basic street rebuild. However, performance engines need to have bearing clearances checked and double checked with precision measuring tools. If you do use Plastigage, check across the entire bearing to ensure there isn’t a tapered journal.

    “If it’s too tight, everybody knows. Too loose, and you know,” quotes Himley.

    The time-honored formula for determining bearing clearance is .001-inch for every inch of crankshaft main journal or rod journal diameter.

    “Then if you want to go loose, add .0005,” suggests Lake Speed, Jr., certified lubrication specialist at Driven Racing Oil. “And for tight, take away .0005. This is on the gross. So, for a 2.5-inch main bearing, standard is .0025, loose is .003 and tight is .002.”

    Loose is fast?

    Mahle Aftermarket, which manufacturers Clevitte 77 bearings, offers a slightly different starting point with .0007 to .001 per inch of shaft diameter, and adds an option of an extra .0005 for performance engines.

    “Then, you can work down from there as experience dictates,” says Bill McKnight, lead trainer at Mahle Aftermarket. “It boils down to weight and grade of oil. If you’re going to run a 0 or 5W oil, you don’t need the extra .0005.”

    First step is measuring the crankshaft main and rod journals. Advice for selecting, using and accurately reading micrometers can take up an entire story. Get familiar with your tools before tackling an important engine project. The key to using a mic is not to tighten it too much on the shaft surface. When measuring the crank, stay away from oil holes and take multiple measurements from different angles to determine if the journal is out of round or tapered.

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    Transfer the main journal measurement to the dial bore gauge and zero the dial indicator. Torque down the main caps with the bearings in place and position the dial bore gauge inside the bearing bore at 90 degrees to the parting line before reading the clearance on the dial indicator.

    Running a little on the loose side has often been a preferred option for engine builders. Fifty years ago, “loose is fast” was a common belief. At the very least, this tactic extended the lives of racing engines—although we’re just now starting to really understand why. More on that thought later. But, first, why are tighter clearances becoming more of the norm in race engines?

    “In most cases it’s easier to do pushups on your palms and not on your fingertips,” says Speed.

    That analogy refers to the hydrodynamic wedge that moves the shaft off center and keeps it from contacting the bearing surface. A low-viscosity or light oil will be squeezed over a greater area of the bearing surface when there is a tighter clearance.

    Checking the rod bearing clearance is conducted in a similar manner. Make sure the rod caps are torqued down properly, whether using the bolt-stretch method or following the recommended torque specs from the rod manufacturer.

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    Again, transfer the measurement from the crankshaft rod journal to the dial bore gauge and zero the dial indicator. Then check the clearance inside the rod bearing shell, measuring 90 degrees from the parting line.

    “Today’s oils have more load-carrying capacity than older oils,” explains Speed. “You’re not concentrating the load in a smaller area; you’re spreading it out and actually reducing the amount of load per square inch. That combination actually frees up more horsepower than a looser clearance and heavy oil in the same engine. A lighter viscosity oil frees up drag on the pump and there’s less drag on the ring pack—all without compromising the bearings.”
    Last edited: Dec 15, 2020
  5. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    not top quality but Ive used the tools for decades

    Engine connecting rod and main bearing clearances have tightened in recent years for many racing applications, but there are caveats to this trend that engine builders need to acknowledge before bending traditional guidelines, especially when adapting those practices to street engines.

    Other factors that need to be considered when setting bearing clearances include the engine-operating environment, cylinder, block material, and oil system.

    “If you’ve got a circle track engine running 40- or 50-lap features, you have to take into account the oil temperature,” says Himley. “Some guys get up to 260 and 270 degrees.”

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    Precision tools are needed to take bearing-clearance measurements. Practice is also needed to properly read the measurements. This dial-bore gauge has a feature that allows it to move quickly and easily between bearings.
    Last edited: Oct 16, 2020
  6. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    Not all engine bearings are the same, and price is not the best way to decide which bearings are best for your application. A quick jog through this story will answer many unanswered questions.

    Story and photos by Jeff Smith

    It was a great day on the dyno. Our 502ci big-block Chevy was singing. It had just picked up a solid 50 hp from a cam and valvespring swap and power was about to break into the 600 hp range. We had previously run this engine with a supercharger, and it had made an awesome 830 hp. The next step was to tune this new normally aspirated package before bolting the blower back on the motor. That’s when it all went sour.

    A backup run on the dyno lost 40 hp, and we knew something was wrong. A quick check revealed shiny metal bits in the oil. We’d lost at least one bearing – maybe more. We cut the oil filter open and our suspicions were confirmed. It was time to take the engine apart.

    The teardown revealed that Number Two main had failed. This was the second big-block that we had worked on in less than a year that had killed Number Two main bearing, and we began to suspect an assembly issue. But a discussion with bearing expert Bill McKnight of Mahle Clevite revealed some information that was so interesting and important for part-time engine builders that we felt we had to pass this along. While we knew there was a difference in price between production style “P” bearings and their high performance cousins, we didn’t know the whole story. And if you build engines, this is important information beyond just the fact that better bearings are more expensive and worth the extra investment. We also learned new information about when production bearings are worthwhile – and when they aren’t.

    The only right way to assemble an engine with new bearings is to not only choose the correct bearings but also spend the time to carefully measure everything with precision tools. There are several different types of engine bearings, and choosing the right one can make all the difference.

    In the case of our big-block, this was a brand new ZZ502 crate engine from Chevrolet Performance. Rated at 500 hp, we had bolted on a supercharger and pushed the power past 830 hp. In performing our Rat autopsy, we learned that these crate engines are assembled using production style aluminum bi-metal bearings. The shell is steel, but the bearing material itself is an aluminum alloy. According to McKnight, nearly all production line engines are now fitted with aluminum alloy bearings not just because they are inexpensive, but also because European laws ban the sale of production car engines with lead alloy bearings. This is understandable as it eliminates the potential for scrapped engine parts putting lead into landfills. When engines are built along production engine assembly lines, these are the bearings that are often employed.

    McKnight said there is nothing wrong with using bi-metal bearings in the ZZ502 application as long as the power remains around 600hp or less. However, when we pushed our engine past 830 hp, crankshaft deflection (even with the ZZ502’s forged steel crank) can cause contact with the main bearings. Scat crankshaft owner Tom Lieb told us that when big-block Chevy bearing failures occur, he most often sees them in mains 2 and 4.

    We also spoke with renowned engine builder Jon Kaase about this phenomenon, and he believes this may be due to where the thrust bearing is located on a big-block Chevy. Because the thrust is located in the rearmost position, leverage from increased power levels tends to load the Number 2 main bearing. To reinforce his point, Kaase points to the big-block Ford that rarely has main bearing problems. Kaase says this is because the Ford places the thrust bearing in the center main, which shortens the leverage arm and therefore reduces the bending moment that is applied to the crankshaft at high horsepower levels. He says that he often sees production 429/460 Fords with 2-bolt mains make over 900 hp with no problems.

    Regardless of why big-block Chevys tend to do this, we wanted to know more about the difference between bi-metal aluminum bearings and their high performance tri-metal cousins. We initially thought that aluminum bearings must be softer than tri-metal bearings, but the truth is exactly the opposite. According to McKnight, bi-metal bearings have virtually the same load carrying capacity as tri-metal versions, but the aluminum alloy is much harder. This is intentional because bi-metal bearings are engineered to last decades in a production engine.

    This durability is a great asset, but it comes at a price. For high-output applications, McKnight says engineers design the bearing softer to “wipe” when extreme loads push through the oil film between the bearing and the crankshaft journal. When this wiping occurs, the softer tri-metal bearing will absorb the load and the bearing material will deflect or wear off. Harder bi-metal bearings are less forgiving. Instead, a portion of the bi-metal bearing face can peel away and begin a microscopic micro-welding process between the bearing and the crank journal.

    Micro-welding can be described as high heat and load removing small bits of aluminum from the bearing surface. Once this process begins, the crank quickly peels away portions of the bearing. This is what we saw with the Number Two main. The metal immediately progresses into adjacent rod bearings and is immediately destructive.

    It’s important to mention here that this is not a condemnation of the ZZ502 crate engine or its aluminum bi-metal bearings. The bi-metal bearings in that engine will last a long time at near-stock power levels. Chevy has tested this engine for durability and found the bearings are fine at that power level.

    We pushed that engine over 50 percent past its intended power levels and abused it beyond its intended capabilities. Had we understood the risk with bi-metal bearings, we would have replaced them with tri-metal versions and our sources all agree that the engine would have survived with no problem with perhaps only a mild wiping of the Number Two main.

    Rod and main bearings can be condensed into two basic categories – production style bi-metal aluminum versions (left) and the more performance oriented tri-metal bearings constructed of a lead-copper alloy with a very thin lead-tin overlay (right). These different bearings may appear to perform the same job, but their performance characteristics are completely different.

    There are many companies making both stock replacement and high performance bearings with the major players being Mahle-Clevite, Daido, Federal-Mogul (Speed-Pro), and King, among others. In talking with King Bearing’s Ron Sledge, we discovered that King makes three different styles of bi-metal bearing. The standard replacement bi-metal bearing uses an AM suffix. The SI suffix bi-metal employs 4 percent silicon and is suitable for daily driven, high mileage engines and mild performance applications.

    Sledge said silicon works as a polishing agent for nodular cast crankshafts. If you were to look at the bearing journal surface of a cast crankshaft under a microscope, it is full of peaks and valleys. Nodular cast cranks contain tin ferrite pockets that are ruptured during grinding and polishing, causing the formation of jagged edges. The silicon in the SI and HP materials acts as a polishing agent to round off the jagged edges. The King HP suffix bearing is the performance version bi-metal and could be used in a small-block Chevy, for example, making 550 hp or less. According to Sledge, using the HP bi-metal bearing will be an advantage in conformability. Above this power level, you would want to use a tri-metal bearing.

    According to Scat Crankshaft owner Tom Lieb, he really likes the King SI bi-metal bearing for non-racing, mild performance cast cranks applications because with these bearings he has had far fewer problems than other bi-metal bearings. As soon as he converted to the King SI bearing, his bearing-related crank issues dropped nearly to zero.

    Tri-Metal Bearings
    So what is a tri-metal bearing? This style of bearing is not really new. It has proven its design as an ultra-high performance bearing for decades. Starting with a steel back for stability, the base is a copper-lead alloy as the second layer with a very soft, electroplated lead-tin alloy as the top layer. This is a generic description with each company modifying this basic layout to a specific formula. The two upper layers consist of very soft materials that will deform and accommodate deflection in the rotating journals that naturally occur in highly-stressed engines.

    This is important because during engine operation what keeps bearings alive is a thin film of oil that creates a hydrodynamic wedge of lubrication that both lubricates and cools the bearing. While softer bearings are more likely to absorb crank deflection, this comes at the price of a shorter overall lifespan compared to a bi-metal bearing. But in the case of high output performance engines, their lifespan between rebuild is far shorter than a traditional production engine that could easily be asked to support 200,000 miles.

    If you are dealing with bearings from an unknown source, many factory aluminum bearings are stamped with an “A” or “AU” prefix that quickly informs you this is an aluminum or bi-metal bearing.

    As you may already know, tri-metal bearings come in many different and specific forms. Crankshaft companies have known for years that creating a large fillet radius between the crank journal and the vertical portion of the counterweight increases the crank’s strength. This narrows the bearing surface area slightly, requiring a narrower bearing. Most bearing companies refer to this style with an N somewhere in the part number.

    The most common application for N bearings is for connecting rods. These bearings come with a chamfer on one side that dictates an upper and lower marked insert. This is critical because if these positions are reversed (an upper bearing shell installed in a rod cap, for example) the chamfer will be located on the wrong side and the 90-degree edge could contact the fillet area of the crank journal. While this is rarely fatal to the bearing, it’s an indication that the engine builder was not paying attention. Other important variations on the traditional tri-metal performance bearing include under- and over-size options that allow the builder to customize bearing clearances. Even half-shells can be used to make very slight bearing clearance adjustments.

    Coated bearings are another option available to the engine builder. For example, Speed-Pro offers a DuroShield coating that is only 0.0003-inch thick but offers an added layer of protection. This polymer coating has the ability to absorb oil to improve lubricity and potentially reduce friction. There is far more information on bearings, coatings, and different chemistries relating to bearings, but our main focus here is to identify the difference between tri-metal and bi-metal bearings. You can decide whose product works best for your application. Talking with professional engine builders will often point you in a specific direction.

    The classic recommendation for rod and main bearing clearance really hasn’t changed much in the last 40 years. The standard rule is 0.001-inch per one inch of journal diameter. This is a standard that still works, but with advances in oil quality and viscosity, these recommendations are becoming more specific. The most important factor to remember when establishing bearing clearances is to match your intended oil viscosity to a bearing clearance. If the engine is already assembled, then you are forced to choose the viscosity that best fits the existing clearance. King Bearing’s Ron Sledge has created a chart that lists clearance recommendations based on engine oil viscosity. This is a generic chart and also refers to main bearing journal diameters less than three inches in diameter. Generally, journal diameters larger than three inches require more clearance. Examples would be Olds or Pontiac engines with the larger main bearings.

    These are Speed-Pro performance tri-metal high performance rod and main bearings. Some people mistakenly think there is something wrong with these bearings because of their odd coloration. Federal-Mogul does not flash coat its tri-metal performance bearings, which is why the bearing looks like this. This is completely normal appearance for a tri-metal bearing.

    Oil viscosity plays an important role in bearing clearance because tighter clearances will demand a thinner oil in order to provide the necessary protection. Another way to think about bearing clearance is to consider the three critical aspects of clearance: bearing load capacity, oil flow, and oil temperature. Load capacity generally peaks with tighter clearances, but minimal clearance reduces oil flow (generally expressed in gallons per minute, or gpm). Tighter bearing clearances also increase localized oil temperature since the oil flow has been reduced. So clearances become a balancing act between all three components. This generally falls, as is indicated in King’s bearing clearance chart, around 0.0020 to 0.0025-inch as an acceptable clearance for street engines.

    With the trend of high quality, thinner viscosity race oils, there is a solid case to be made for tighter clearances that can take advantage of slight power gains from reduced windage in the crankcase from less oil but also improved power from reduced pumping losses due to the lighter viscosity.

    An oil viscosity test performed by Steve Brule at Westech Performance a few years ago found that high quality 0w-20 race oil produced an average of 3 hp more compared to a straight 30w street oil and an average pressure drop of nearly 8 psi compared to a 20w50 race oil. Ironically, the average power between the 0w20 and 20w50 oils was a mere 0.5 hp despite the difference in pressure, so gains are small when contemplating power increases through oil viscosity.

    Oil pressure is another aspect of engine performance that is changing. The old standby of 10 psi per 1,000 rpm of engine speed is now considered to be outdated. According to Sledge, NASCAR engines are now running 9,000 –plus rpm with oil pressure in the 40 psi range.

    Performance bearings come in multiple sizes. For performance applications, often a 1X bearing can be used to add 0.001-inch additional clearance. A -1 bearing will reduce the clearance by 0.001-inch. When mixing shell halves for custom clearances, always place the thicker shell half in the loaded side of the housing. In the case of rod bearings, this would be in the connecting rod side. Over- and under-size bearings halves should only be used in conjunction with a standard bearing half. Never combine a +0.001 with a -0.001 half.

    If you think about it, pressure is just the indication of resistance to flow. Pressure is important, but it’s clear that 50 psi for most street performance applications would be more than sufficient. This also means that oil pressure at idle does not have to be 40-plus psi. At idle, the engine only has to make enough power to spin itself and any attending accessory drives. With this minimal load, oil pressure at 10 to 20 psi would be more than sufficient. This is why nearly all OE manufacturers are switching to 5w-20 or 5w30 oil for production engines.

    Thinner oil requires less power and therefore results in better fuel economy and power. We once asked Kaase what the typical oil pressure was on an 800ci Pro Mod style mountain motor. Kaase says he gears his dry sump oil pressures for peak rpm and they fall wherever they may at idle. Often this means the engines may only make 5 psi of pressure at idle, which he feels is more than sufficient.

    We’ve only barely touched on several aspects of engine bearing applications and usage, but perhaps we’ve piqued your interest and you can take the next few steps in determining what’s best for your specific application. It should be obvious that the selection of a main or rod bearing for a daily-driven small-block is going to be quite different than one for a 9,000 rpm NHRA Competition Eliminator drag race engine. But at least now it’s clear that not all bearings are the same.

    King Bearing Oil Clearance Chart

    Oil Viscosity Rod Bearing Clearance Main Bearing Clearance

    20w / 5w20 < – 0.0021 < 0.0020

    30w / 5w30 0.0021 – 0.0026 0.0020 – 0.0025

    40w / 10w40 0.0026 – 0.0031 0.0025 – 0.0030

    50w / 20w50 0.0031 – > 0.0030 – >


    reading as many links related to any questions you have as you can!
    and read through the links and sub links

    Fowler Full Warranty Extender Dial Bore Gage Set, 52-646-400, 1.4-6" Measuring Range, 0.0005" Graduation Interval: Bore Measurement Gauges: Industrial & Scientific
    Fowler Full Warranty Extender Dial Bore Gage Set, 52-646-400, 1.4-6" Measuring Range, 0.0005" Graduation Interval: Bore Measurement Gauges: Industrial & Scientific

    6 Pcs Professional Premium Outside Micrometer Precision Machinist Tool Set 0-1"/1-2"/2-3"/3-4"/4-5"/5-6" Measuring Range Included 0.0001" Graduation: Industrial & Scientific
    6 Pcs Professional Premium Outside Micrometer Precision Machinist Tool Set 0-1"/1-2"/2-3"/3-4"/4-5"/5-6" Measuring Range Included 0.0001" Graduation: Industrial & Scientific

    Often after measuring a complete set of eight rod bearings, you may find a set that is a little tighter clearance and one that’s a little looser than you’d like. We have found that merely switching the loose for the tight and measuring again will bring both tolerances in closer to the desired clearance. This can be attributed to minor housing bore differences and different crush characteristics between the rods.

    If you look closely at a high performance set of rod bearings, the inserts may be marked “upper” and “lower.” This is because the bearing insert has been narrowed on one side to clear the radius against the cheek of the crank. Be careful to always place them in their proper position to prevent the bearing from rubbing on the crank fillet radius. When inserted properly, the narrow side of the bearing will be on the same side as the chamfer on the rod and cap.

    When customizing bearing clearances, you can mix half-shell thicker or thinner bearing halves to establish the desired clearance. For example, to decrease rod clearance by 0.0005-inch, use a -1 (0.001 less clearance) in the rod and then measure the clearance again. Use only the same style bearings when mixing half sizes and never mix manufacturer bearings in the same housing bore.

    Most leading bearing companies now offer ¾ groove style main bearings. This extends the bearing’s oil groove from the upper bearing past the parting line to ensure a better supply of oil to the loaded portion of the bearing in the cap. Full groove bearings should never be used in a performance application because this reduces bearing surface area and increases the load on the remaining portion of the bearing.

    Only measure bearing clearance 90 degrees to the parting line. All engine bearings are designed with additional clearance closer to the bearing parting line. This taper prevents bearing movement due to cap walk that could potentially wipe oil off the crank journal.

    Anytime a set of rod bolts are pressed into place, the rods should be resized to accurately maintain not only the proper inside diameter but also concentricity. Changing from bolts to studs in main bearings will also change the clearance due to different load characteristics. Changing to main studs often does not require line honing, but you should measure the clearance as it could change.

    There are some enthusiasts who will argue this point, but we don’t consider Plastigage to be a precision measurement device. This tool can be used when nothing else is available, but in our experience, this tool is extremely inconsistent with variations of well over 0.001-inch compared to using a calibrated dial bore gauge and a micrometer.

    This is the aluminum main bearing we pulled from our big-block Chevy engine when it began to destroy the bearings. Note how a portion of the bearing is severely scratched. This is where the aluminum alloy was pulled away by the crank. That’s the metal we found in the oil. A softer tri-metal bearing would have been worn down but would not have dumped bearing material into the oil – unless it was severely distressed – at which point you have other problems.


    Daido Metal, USA;

    Federal-Mogul (Speed-Pro); 248/354-7700;

    King Bearings; 800/772-3670;

    Mahle Clevite; 800/338-8786;

    Melling Automotive Products (Dura-Bond); 775/883-8998;

    Scat Enterprises; 310/370-5501;
    Last edited: Dec 16, 2020
  7. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    yes Im fully aware most people don,t bother to read the links and sub-links
    UNTIL they have expensive parts fail.
    but for the few people on the web site that might prefer spending less time and cash replacing expensive failed parts,

    and financing the machine shop owners vacations and paying his mortgage... I post them anyway
  8. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Nice Read today Grumpy.
    All LS engines have been built with Aluminum Bi Metal bearings including B8 & B15.
    Never liked Bi metal.

    Tri Metal Babbit bearings my pick.
    Vintage Indium Tri Metal Vandervell also.
    Vandervell still around in Europe. They do High end IRL Race cars only now.

    Crank deflection...bad.
    Hard to believe BBC.
    Non Issue Pontiac 455
    Crank is huge as you know.

    You know Crank deflection takes places in LS High Torque High Hp.
    No exotic alloy crank. CAST MOST.
    Tiny mains.
  9. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    Measuring Main Bearing Clearance

    Pat Mancuso - -

    Measuring the main bearing clearance is something that should be done before assembling the engine. This is commonly done with plastigage, but that method is not very accurate. Here's how to measure the clearance with an outside micrometer and a dial bore gauge

    The obvious answer is to measure the main journal on the crank, and measure the diameter of the main bearing, and subtract. Not so fast. The problem with that method is that you're using two tools to do the different measurements, and there is no way to know what the error was in measuring each.

    The textbook measurement for the main journals on a SBC 400 is 2.6500". Let's say you measure 2.6500 on the journal with the outside micrometer, and 2.6525 with a dial bore gauge on the bearing. That sounds like you've got 0.0025 clearance, right? Wrong. Let's say the error rating on the micrometer and on the bore gauge was +/- 0.0005. That means the journal could have really been 2.6505 to 2.6495, and the bearing could have been 2.6530 to 2.6520. Taking the worst pairs, 2.6505 - 2.6520 = .0015 and 2.6495 - 2.6530 = .0035, so your clearance is really somewhere between 0.0015 and 0.0035 once you figure in the potential error. That's a pretty wide range for the tolerances you need to check.

    How do you get around that without spending a fortune on more accurate measuring tools? Easy, always use the same tool to measure with and you eliminate half of the error uncertainty. The basic idea is to measure the journal with the outside micrometer, then zero the bore gauge inside the micrometer, then use the bore gauge to measure the bearing. The actual measurement doesn't matter, you end up seeing only the difference in the measurements, which is the clearance measurement you wanted to check in the first place.

    Going through the steps, measure the crankshaft journal with the micrometer, zero the bore gauge against the micrometer, and then measure and get .0025 (+/- .0005 error) which means your actual measurement is between .0020 and .0030. Much better than the previous method.

    When using this method, you must be sure to measure each journal and re-zero the bore gauge before measuring the corresponding bearing. Measure the #1 journal, zero the bore gauge, use the bore gauge to measure the #1 bearing, measure the #2 journal, zero the bore gauge, use the bore gauge to measure the #2 bearing, etc.

    To get an accurate measurement of the bearing diameter, make sure the bearings are installed properly in their correct location, and the bolts are tightened to the final assembly torque specifications.

    Using the outside micrometer on the main bearing journal:


    Zeroing the dial gauge against the micrometer measurement:



    Reading the clearance at the bearing:


    Showing .0030 difference:

    Blueprint Series: Measuring And Setting Bearing Clearances

    By JEFF SMITH JULY 13, 2018

    If we had to choose one operation that epitomizes the process of engine blueprinting – we can’t think of a better one than setting bearing clearance. This goes far beyond slapping a set of new bearings in the main saddles, torquing the main caps in place, and hoping the crank turns over. Blueprinting clearance means establishing a clearance that is your target number and working the components until this number is achieved. Anything else is just bolting an engine together.

    We won’t get into establishing specific clearance goals here, because that has been previously covered by EngineLabs. We can offer the standard advice that is tried and true – multiply the crank journal diameter by 0.001-inch. As an example with a small-block Chevy main journal of 2.200-inch – then an oil clearance of 0.0022-inch would be a great place to start.

    This discussion will focus on main and rod bearings in a mild, street-driven performance engines that might see occasional high-RPM use, like at the drag strip. Perhaps the first bit of information worth mentioning is that this is the total clearance around the circumference of the bearing. So in the case of a 2.500-inch main bearing with a vertical clearance of 0.0025-inch, this establishes there is only 0.00125-inch clearance between the crank journal and the main bearing at the top and bottom. Under maximum load, the oil is squeezed into a very tiny area of clearance measured with five digits to the right of the decimal point–perhaps as tight as 0.00025-inch. The remainder of the clearance is found on the unloaded side–the top side of the main bearing or the bottom side of a rod bearing.

    In order to do this job properly, you will need some accurate measuring tools. A minimum of a 2-to-3-inch inside micrometer and a dial bore gauge are necessary. Be sure the micrometers and dial bore gauges will measure down to 0.0001 inch. Cheaper tools often only measure down to 0.001 inch. This is not precise enough by a factor of 10. You will also need a torque wrench and sockets.

    The large amount of bearing clearance on the opposite side of the load is used to feed oil between the journal and the bearing, which is why producing sufficient clearance is so important. It is this dynamic loading of the bearings that reinforces why attention to detail is so important. There are other considerations such as bearing crush, eccentricity, and bearing materials that demand close scrutiny, but we will focus on how a DIY builder can create professional results by using high-quality measuring tools and working carefully.

    We will make some very important assumptions that the block and crankshaft have either been machined or carefully measured to ensure they are straight, with minimal taper, so that our measurements will pay off with a happy engine when assembled.

    Creating the desired bearing clearance starts with accurately measuring the journal diameter. In this case, we’re measuring a big-block crank main journal. The only accurate way is to use a micrometer that measures down to 0.0001-inch.

    The first order of business is to measure the crankshaft. We will need a quality outside micrometer, a notebook to record the readings, and a clear, clean work bench. The crank should be clean and ready for assembly. Assuming we’re working with a V8 engine, it’s important to measure the main journals in two locations and record both. If you are really fastidious, it’s a great idea to measure for taper across the journal as well.

    Once a journal diameter is established, there are two ways to go about setting up your dial indicator to measure the inside diameter of the bearing housing.

    • [​IMG]
    • [​IMG]
    With the micrometer at a specific journal diameter, use this to zero the dial bore gauge (left). We placed our mic in a bench vise to hold it firmly, protected by a thick rag. Setting the dial bore to zero requires attention-to-detail to make sure the zero is accurate. If you induce an error at this stage, every other measurement will be in error. Next, use the dial bore gauge to measure the inside diameter of the housing bore, in this case, the number two main journal that has been torqued (right). For maximum accuracy, measure bearing clearance only in the vertical. Also check for taper in the rod. We had a poorly resized used rod that had 0.0015-inch taper. This is caused when the rods are not switched on the mandrel and only honed from one side. This creates a taper or bell-mouth in the rod big end. So always check for taper on rebuilt rods.

    One way is to set the outside micrometer to a specific journal diameter. Let’s use a 454ci big-block Chevy as an example. With a 0.010-inch-under crankshaft, we measured the number three journal at 2.7387-inch. This is exactly 0.010-inch undersize. We can set the dial bore gauge to read zero at this point and then install and measure the inside diameter of the bearings in the number three main.

    The second procedure saves time but could introduce a math error. This process measures all the journals. Then the builder sets the dial bore gauge to one journal size and performs the math to adjust the clearance from the dial bore gauge for the different housing bore diameters. As an example, if we set the dial bore gauge to the 2.7387-inch diameter of journal three, then we would add or subtract the difference of varying sizes of the journals to produce the actual bearing clearance. If the journal is larger than our standard by 0.0002-inch, then we would subtract 0.0002 from the dial bore gauge reading for clearance for that main bearing.

    We made a simple oil pump adapter for this big-block and mounted it using the stud and nut we planned to use in the engine. We measured clearance before and after torquing this in place and discovered the clearance increased in this situation by .0008-inch!

    As an example, if we installed 0.010-under bearings and measured the clearances and all was right with the world, the dial bore gauge should read +0.0025-inch (our desired clearance) for all five main journals. But this only happens on TV car shows and magazine engine articles. In a big-block that we recently assembled for a friend, the crank main journals measured as follows:

    Main Journal

    Journal Diameter

    Actual Undersize
















    None of the crank main journals measured the same and only number three was the technically correct 0.010-inch undersize. Budget and time limitations prevented us from grinding this crank 0.020-under. Instead, we had to deal with this and use multiple size bearing shells to bring the clearances as close as possible.

    Let’s first address the clearances for the 0.010-under number three. Measuring the actual clearance using 0.010-under Federal-Mogul bearings, we came up with 0.0027-inch. This was slightly more than our ideal 0.0025 spec but acceptable. The other four created either too much or too little clearance using just 0.010-under bearings.

    Most race bearing manufacturers will offer bearings in +/- sizes. For example, Federal Mogul offers main and rod bearings in 0.001-inch under and oversize versions within the lineup of standard, 0.010-inch, 0.020-inch undersized bearings. That allows you to set the clearance by adjusting, even with half-shells.

    Some performance bearing companies like Federal-Mogul offer optional bearing sizes such as 0.001 undersize or oversize inserts that make it much easier to set an ideal clearance. In our case, we needed 0.011-inch undersized on some of the journals and 0.009-inch-undersized bearings for the front two. Federal-Mogul offers these and saved our bacon. This allowed us to increase or decrease the clearances to get closer to our ideal. While mixing half-shells is acceptable practice, never mix shells with more than 0.001-inch spread and always stay within the same manufacturer. In other words, never mix a 0.009 bearing shell with an 0.011-inch version.

    One down side to performing all these customized clearances is that we were faced with purchasing two (and in our case, three) sets of main bearings for one engine. So do all measuring before you buy the bearings. The same is true with rod bearings.

    It is accepted practice, for example, to mix one 0.010-under shell with an 0.011-under bearing half on a specific rod or main journal to achieve the desired clearance. Never mix bearings of different manufacturers and never mix bearing halves that are more than 0.001-inch different in size.

    The best way to fix this would have been to have the engine align honed to establish the proper housing bore diameter. In our case, the engine had to go back together due to deadlines beyond our control so we did the best we could. The final 0.0035-inch clearance is well within factory tolerances, but it is also 0.0005 inch wider than we would prefer. For a mild street motor, this was acceptable. Another reason this will work is that as the thrust bearing, this additional clearance will provide more than enough oil to properly lube the bearing’s thrust surfaces.

    These are Federal-Mogul 0.010-under rod bearings. Note the stamp “L” or “U” on these shells along the upper stamping. The “L” means this shell must be installed in the lower half of the connecting rod so that the bearing’s chamfer will be on the same side as the crank radius. The “U” obviously is the upper insert. If the bearings are inverted, the chamfer will not be adjacent to the crank journal radius and the bearing may rub. This isn’t a major issue, but certainly something to watch carefully.

    It’s also important to point out that housing bore diameter, whether it be the rods or mains, have a big effect on bearing clearance. Incorrect clearances are commonly blamed on the bearings when the reality is the housing bores are improperly sized. When combined with inconsistent crank journal diameters, this tolerance stack-up is the real culprit in nearly all clearance issues. Measuring these parts is the only way to know for sure.

    At some point in the Blueprinting series, we will also look at the accuracy of the measurement tools you are using. If your measuring devices are not accurate to at least 0.0002-inch, the actual numbers may not be an accurate reflection of what is really there.

    When bearings are installed in the connecting rod, always use some type of rod vise to clamp across the cap parting line. This prevents damage to the rod when the bolts are tightened. If you don’t have a rod vise, use a bench vise with aluminum inserts in the jaws to prevent damage to the connecting rod.

    It’s also important to point out that bearing clearance will dictate engine oil viscosity. We will have to over-generalize here, but tighter clearances demand thinner oil while wider clearances will need a higher viscosity oil to establish the proper oil-film thickness to prevent abnormal wear.

    Most of the details in this story relate to employing common sense and accurate measurement techniques. Accomplish both of those tasks and your engine will live a long and powerful life.

    If you really want to get down into the tiniest of details, you can test bearing thickness variations. You will need a round bearing adapter for your micrometer as shown (we found ours through Grainger). However, our experience is that accuracy of measurement becomes more of an issue than variations in bearing thickness. In other words, can you accurately measure to 0.0002-inch?

    Last edited: Jan 31, 2021
  10. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member


    One size certainly doesn’t fit all when it comes to bearing oil clearances. Old school engine builders usually prefer to build an engine “loose” and run a heavier oil such as 20W-50 or straight 50 so the bearings can handle more crank flex under load. This works well in drag racing because the crankshaft experiences a lot of twisting and flexing. A little extra clearance is also good in a dirt track engine because the engine runs in a very dirty environment. Yet NASCAR engines are often built with much tighter bearing clearances (.0015˝ to .002˝) because they are using low viscosity synthetic racing oils.

    Most rod and main bearings run best with .0007˝ to .001˝ of clearance for every inch of crankshaft journal diameter, or .0015˝ to .002˝ inches of oil clearance for a 2-inch diameter shaft – but these numbers will vary with the oil viscosity and application. Typically, an engine builder might add .0005 inches of additional clearance for a performance motor over what he would normally use with a stock build.

    One bearing manufacturer recommends the following oil clearances based on the viscosity of the oil used in the engine:

    • .002 for 20W and 5W-20 oils

    • .0025 inches for 30 and 5W-30 oils

    • .0025 to .003 inches for 40W, 10W-40 oils

    • .003 to .0035 inches for 50W and 20W-50 racing oils.

    As for oil pressure, you only need enough to keep the oil film between the bearings and the journals. The old rule of thumb of having 10 lbs. of oil pressure for every 1,000 RPM is still valid, but some engine builders are going with less to reduce parasitic horsepower losses at the oil pump. An oil pressure reading of 60 PSI at idle may look great on a gauge, but it’s overkill for what most engines need. Most engines don’t need more than about 10 PSI at idle, and can get by with 5 to 7 PSI.

    The amount of oil flow will also vary with the application. Aluminum engines tend to run looser and flow more oil than cast iron engines due to the difference in thermal expansion rates. Ditto for aluminum rods. An engine that has piston oilers or supplemental oil feed lines for the rocker arms and valve springs will require more flow and a higher volume oil pump.

    Bearings should be installed dry in the bores, then lubricated on the journal surfaces with assembly lube or oil. The engine’s oil system should also be primed before initial start-up to prevent damage caused by a dry start. If you hear any rapping or knocking noises once the engine is running, shut it off. There may be a loose rod cap or excessive bearing clearances that somebody overlooked.

    B&R Racing likes this.

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