main cap fit in block?


Staff member
I picked up my 4 bolt mains 454 from the machine shop. Bought block on ebay and had in shop for
magnafluxing and cleaning block was baked to clean.
tried to reinstall main caps and they are not seating to the block. afraid to use force is this normal something that will be taken care of when I
take the block back for boring and line honing


properly fitted main caps won,t drop in effortlessly they are designed with a MINIMAL .003 to .004 interference fit so they don,t have a tendency to move around once securely tightened into place, Id suggest having the block sonic tested for cylinder wall thickness and checked for cracks, its unfortunate but theres a significant amount of TRASH BLOCKS being sold to recover at least some value at the expense of other on ebay
BTW the main caps should be numbered and stamped so you know which cap goes in each location and which side faces forward, and rods should be stamped to match cylinder location, many but not all factory main caps have a arrow cast in to indicate which side faces forward


together correctly

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the better machine shops will suggest you pin the main caps to prevent movement, or or sleeve the main cap studs or bolts, to insure the caps can't move around on the block under high stress loads





keep in mind both the main caps base and the area in the block must be machine precisely parallel , and there is a slight interference fit into a slight recess in the block on most engines to help prevent the caps moving once tightened into place by the main cap bolts or main cap studs. on some performance engines its fairly common for a hollow sleeve,s inserted 1/2 its short length, into the block and 1/2 its length into a shallow recess into the main caps, too fit into matching recesses around the main cap studs , this locates and prevents lateral movement of the main caps








A simple modification to the upper thrust bearing may be beneficial in some engines. Install the upper thrust bearing in the block to determine which thrust face is toward the rear of the engine. Using a small, fine tooth, flat file, increase the amount of chamfer to approximately .040" (1 mm) on the inside diameter edge of the bearing parting line. Carefully file at the centrally located oil groove and stroke the file at an angle toward the rear thrust face only, as shown in the illustration below. It is very important not to contact the bearing surface with the end of the file. The resulting enlarged ID chamfer will allow pressurized engine oil from the pre-existing groove to reach the loaded thrust face. This additional source of oiling will reach the loaded thrust face without passing through the bearing clearance first (direct oiling). Since there may be a load against the rear thrust face, oil flow should be restricted by that load and there should not be a noticeable loss of oil pressure. This modification is not a guaranteed "cure-all". However, the modification should help if all other conditions, such as surface finish, alignment, cleanliness and loading are within required limits.
engine building is about 75% technology and science and 10% art, and 15% skill gained through experience,over time.
doing it successfully mandates the person involved, in the process, can accurately observe, think, and recognize problems,
and if required, stop and think up solutions.

yes it involves knowing how components are intended to function and accurate precision measurement and access to some specific tools and a skilled well equiped machine shop you can trust, is a mandatory factor and skill set.
so many guys seem to be under the absurd impression that any and all solutions,
to any and all problems,
invariably involve, throwing away some component,
and installing some new out of the box part,
instead of acting logically and actually measuring,
thinking and potentially custom clearancing or at least reading the instructions,
and as most experienced engine builders will tell you ,
almost nothing you can buy fits and functions correctly in out-of-the-box, condition
,too near its true potential.after its ,
carefully, inspected, and correctly clearanced, and fitted.











if you check you'll find that stud girdle use does little or nothing for the individual main cap strength but it does marginally increase main cap stability and block flex.
now the potential difference is probably not worth the expense, in that your generally spending cash that would be better used in the purchase of the stronger aftermarket block casting from a known source like DART.
look through the links and read the sub links
the billet splayed main caps on the aftermarket block is the stronger route, but Id bet 90% of the guys building their first engine think they will save money using the O.E.M. block they already own.....well, until... they add up all the machine work costs and price of parts like aftermarket splayed caps, ARP main studs ,the labor costs from the machine shop, etc. but by that time the machine shop owner is smiling all the way to the bank, and youve just figured out the true cost of that cheaper O.E.M BLOCK

if you have a two bolt main cap block and you don,t want to go to the considerable expense og having the block converted to splayed aftermarket main caps the option of adding a main cap stud girdle, and ARP main cap studs, may appeal to you as it adds a bit more block strength & rigidity, but its not going to be as strong as a aftermarket DART block or the splayed main caps and it can limit the oil pans and windage tray options




OEM main caps on most blocks are cast iron and have a tendency to fracture under lower stress loads than machined steel splayed main caps,that have outer bolts anchored in the blocks thicker outer section, this can be an important consideration if you intend to use nitrous or push an engine well in excess of 4000fpm in piston speeds

the crank or block can have the correct bearing clearance but still be slightly bent or the block may be warped and result in the bearing wear , keep in mind main bearing caps can crack or be improperly machined, this is FAR less common on DART AFTERMARKET BLOCKS


the cap register should require a firm whack with a plastic mallet to seat them into the block[/b]


  • Siamesed Extra-Thick Cylinder Walls: Resists cracking and improves ring seal (minimum .300'' thick with 4.625'' bore).
  • Scalloped Outer Water Jacket Walls: Improves coolant flow around the cylinder barrels to equalize temperatures.
  • Four-Bolt Main Bearing Caps: In steel or ductile iron have splayed outer bolts for extra strength.
  • Crankshaft Tunnel: Has clearance for a 4.500'' stroke crank with steel rods without grinding.
  • True ''Priority Main'' Oil System: Lubricates the main bearings before the lifters.
  • Oil Filter Pad: Drilled and tapped for an external oil pump.
  • Rear Four-Bolt Cap: Uses standard oil pump and two-piece seal - no adapter required!
  • Lifter Valley Head Stud Bosses: Prevent blown head gaskets between head bolts.
  • External Block Machining: Reduces weight without sacrificing strength.
  • Simplified Install : Fuel pump boss, clutch linkage mounts and side & front motor mounts simplfy installation on any chassis.
  • Dual Oil Pan Bolt Patterns: Fits standard and notched oil pans.
  • Bellhousing Flange and Rear Main Bearing: Reinforced with ribs to resist cracks.
  • Note: Does not include cam bearings, freeze plugs, or dowels
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Staff member
when chevy engineers designed the first generation engine block, it was a 265 cubic inch engine design that was soon cast and bored to a larger bore 283 and eventually to a 327-350 displacement, when they decided to increase displacement to 400 cubic inches the basic design with most dimensions remained similar but the space for coolant flow between the cylinder bore was no longer a practical choice so they changed the design to a Siamese bore block where the space between the adjacent cylinder bores was omitted to allow the bore wall thickness to be reasonably solid, and thick enough to prove durable.
remember when chevy engineers design a block they must compromise between the ideal strength design and the least expensive to build design that will function.
a stock cast iron chevy small block engine block will weigh close to 190lbs a 1% reduction in the metal required to cast one is obviously close to 2 lbs, multiply that by the hundreds of thousands of blocks cast and your saving TONS of material and in transportation costs.
most stock first gen small block chevy engines will rarely exceed 5500rpm or make over 350 hp because they are mated to stock automatic transmissions that shift to the next gear,at under 5500rpm.
thin stock castings tend to warp slightly over hundreds of heat and cooling cycles thats why most racers who use STOCK BLOCK castings prefer a good USED block which has already settled to a no longer likely to warp further from its final main cap location after its line honed and deck milled
the pictures below are not a stock production block but the much thicker and more rigid aftermarket DART block
Ive occasionally been asked what you can do too reduce the slack in the timing chain if your blocks been line honed,
to straiten the main bearings and that resulted in a slightly closer crank to cam center-line distance,
that results in a slightly increased slack in the stock timing chain sets.
a negligible amount of metal is generally removed from the main bearing saddles in the block, they usually try very hard to minimize that, metal removal so standard parts still fit,during a line hone , but they do sell slightly tighter timing chain sets to correct excess slack if that's required.


Timing Chain and Gear Set, Original True Roller, Double Roller, -0.005 in., Iron/Steel Sprockets, Chevy, Small Block, Set
for line honed blocks where the crank is .005 closer to the cam
for line honed blocks where the crank is .010 closer to the cam
Timing Chain and Gear Set, Original True Roller, Double Roller, -0.010 in., Iron/Steel Sprockets, Chevy, Small Block, Set





related info ... lock_prep/ ... bject.aspx

Align Boring 101

By Larry Carley

A horizontal boring bar with cutters mounted on it is inserted into the block and then centered in the main bearing or cam bearing bores with support bushings or fixturing. The bar is then turned and advanced to shave metal off the inside of the bores so the inside diameter (ID) of the bores can be resized to the desired dimensions (back to standard size or to oversize).

An alternate method for machining the bores is to use a line hone. A hone uses abrasive stones rather than cutters to remove metal. Line honing typically removes less stock and leaves a smoother finish than line boring, making it well suited for applications where only minimal stock removal is necessary or where a smoother bore finish is desired or required (as in overhead cam cylinder heads where the cam journals have no bearing shells or inserts).

Why Bore?
There are three basic reasons for line boring the main bearing and cam bearing bores in engine blocks. One is to restore worn, out-of-round or damaged bores. If an engine overheats or loses oil pressure, one or more bearings on the crankshaft or camshaft may seize and spin. The resulting damage to the bearing bore must then be repaired by either machining the hole to accept a standard sized bearing or an oversized bearing.

With main bearings, a worn, out-of-round or damaged bore can be restored back to standard ID by grinding or milling the mounting surface of the main caps, bolting the caps back on the block, and then cutting the holes back to their original dimensions.

In the case of worn, out-of-round or damaged cam bearings in an engine block, there are no removable caps. The only option is to enlarge the bores so new oversize cam bearings with a larger outside diameter (OD) can be installed.

Reason number two for line boring a block is to restore proper bore alignment - a process which is often called "align" boring (or honing if a line hone is used instead of a boring bar). As rigid as an engine block might seem, there is actually quite a bit of residual stress in most castings. As a new "green" block ages and undergoes repeated thermal cycles, the residual stresses left over from the original casting process tend to distort and warp the engine. This affects the alignment of the crankshaft and camshaft bores as well as cylinders. Eventually things settle down and the block becomes more or less stable (a "seasoned" block). The bearings as well as the crankshaft and camshaft journals gradually develop wear patterns that compensate for the distortion that has taken place.

Additional warpage can occur if the engine is subjected to extreme stress (like racing) or overheats. If the original crankshaft or camshaft is then replaced without align boring the block, it may bind or cause rapid bearing wear. Likewise, if you're building a high performance engine with close tolerances, you don't want any misalignment in the main bores or cam bores.

The third reason for line boring or honing a block is to correct or change bore centers or bore alignment (as when "blueprinting" a high performance engine). The camshaft and crankshaft should be parallel in the block. If they are not, line boring can correct the misalignment to restore the proper geometry. With performance engines, there may also be a reason to change the centerline of the crankshaft or camshaft slightly to alter the piston or valvetrain geometry.

Line boring will also be required if the original main bearing caps are replaced with stronger aftermarket caps, or the block is being converted from two bolt main caps to four bolt main caps. For best results, four-bolt main caps should be machined in three-steps. First, bore the housings to within .030" of the desired size. Then bore again to within .005" of final size, and finish to size by align honing. Harder honing stones work best on cast iron while softer stones (such as J45 silicon carbide #150 grit) do better on bimetal applications where the block is aluminum and the main caps are cast iron, steel or powdered metal.

Even though the end goal of line boring has remained constant, some innovations in the recent past have made line boring anything but a boring subject.

One of the disadvantages of using a traditional horizontal boring bar is that it tends to sag. This has to be countered by using adequate support so all the bore holes are cut straight and true with no misalignment between holes and no variations in bore size.

One way to eliminate the effects of gravity on the boring bar is to use a vertical boring machine. Rotating the block and bar 90° so the block and bar are straight up and down provides a truer, straighter cut says one manufacturer of this type of equipment. It also saves floor space because the machine has a smaller footprint.

Another way to circumvent the issue of bar sag is to use a 90° right angle cutter attachment on a milling machine. Instead of using a long steel bar to pass single or multiple cutters through the main bores, the 90° cutter is lowered into the space between each main bore, then moved sideways to machine the bore ID. It's sort of like working around a corner. With computer numeric controls (CNC), each hole can be precisely machined to exact dimensions and the centerline of each hole perfectly located and aligned with all the rest. This technique works especially well on large, heavy blocks that may be too long for most boring bars.

OHC Applications
When overhead cam cylinder heads came into widespread use, it quickly became apparent that line boring or honing would be needed to correct a variety of problems (cam bore wear, distortion and damage as well as cylinder head warpage). Aluminum heads can be easily warped by overheating. When the head gets hot, it tends to swell the most in the center area. The head bulges up in the middle causing misalignment in the cam bores. This, in turn, can cause uneven wear in the cam bores, camshaft sticking or even cam breakage.

If an OHC cam won't turn in the head, it means the cam is sticking because either the cam is bent or the head is warped. In the case of a bent cam, the center cam bores will show excessive wear or be worn out of round. If the head is warped (which many aluminum heads are), the head should be straightened BEFORE it is line bored or resurfaced.

Note: Most of the distortion will typically be in the center area of the head. When checking the flatness of the deck, don't just place your straightedge down the center of the head. Also check flatness over the head bolt holes along both sides of the deck.

To straighten the head, bolt the head to a heavy steel straightening plate using shims under the high spots to offset the warpage. Then place the head in an oven that has been preheated to about 500° F. Leave the head in the oven for about two hours, while monitoring the temperature of the head so it is kept at about 450° to 475° F. Use a contact pyrometer to check head temperature, not an infrared pyrometer. You don't want to get the head too hot (over 500° F) because this can soften the head too much. Also, if you don't get the head hot enough (at least 450° F), you may not straighten the head at all! At the end of the oven cycle, pull the head out of the oven and allow it to air cool.

Other methods of straightening aluminum heads include cold pressing the head (risky because you might crack or break the head), TIG welding on the top of the head to pull it flat, and spot heating with a rosebud torch to counter the distortion.

The main goal should be to get the alignment of the cam bores as straight as possible rather than straightening the deck surface. Low spots in the deck can always be built up by welding and machining the head flat (which may require using a head gasket shim depending on how much resurfacing is required). But as a rule, once you get the cam bores straight, most of the distortion on the deck will also disappear.

In the past, a popular fix for OHC heads with worn cam bores was to line bore the head to accept bearing shells or inserts. Nowadays, the more popular fix seems to be cutting the head to accept a cam with oversized journals.

Alignment Checks
The alignment of the crankshaft and camshaft bores in blocks and OHC heads can be checked by placing a straight edge in the bores or along the bore parting lines and using a feeler gauge to check for misalignment.

How much misalignment is too much? It depends on the engine and the application. A light duty passenger car engine is not as critical as a high revving performance engine or a hard-working diesel engine. As a rule, most passenger car and light truck engines call for .002" or less of misalignment between all the bores, and .001" or less misalignment between adjacent main bores. For performance engines, you can reduce these maximum tolerances by half or more.

Another dimension to look at is bore wear. Bore diameters should usually be within .001" of specifications to support the bearings properly, with no more than .001" out-of-roundness if the horizontal dimension is greater than the vertical dimension.

Also check for wear on the thrust bearing surface of the main cap. If worn or damaged, this surface should also be remachined.

Changes In Centerline
When main bores in engine blocks or cam bores in OHC heads with caps are worn excessively, removing the caps, grinding them down and align boring or honing the holes back to size with the caps in place can usually restore the bores. But this will change the centerline of the crankshaft or camshaft slightly, moving it further into the block or head unless corrective measures are taken to prevent this from happening.

If a block is being align honed, the bushings that support the honing bar are usually mounted in the undamaged end journals of the block. Centering pins in the middle of the bar are used to center the bar in the center main bore. Stock removal is usually limited to about .003" or less when honing.

With line boring equipment, pilots are used to position the bar. This allows faster stock removal and does not usually require any oil or lubricant. Changing the position of the bar will change the centerline of the bores and crankshaft.

Many engines can handle a few thousandths variation in the position of the crankshaft centerline but others cannot because of changes it causes in other critical dimensions such as the deck height of the pistons when the crank is at top dead center (which affects compression, piston-to-head clearance and valve-to-piston clearance). Performance engines and diesels are much more sensitive to centerline changes than light duty passenger car engines.

Because main bore alignment is so important, it should be the first thing that is machined on any engine. And it must be done accurately because most of the other critical dimensions center off the crankshaft.
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The Grumpy Grease Monkey mechanical engineer.
Staff member
HERES A PICTURE, main caps are usually cast with a arrow showing the direction they face but seldom number stamped to indicate the correct location in the block and its best to do that during the dis-assembly too insure they go back in the correct location.


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its usually standard practice to lightly stamp the outward facing rods and rod caps and main journal caps with the cylinder number or location they will be or are located in and a matched stamped number on the oil pan rail of the block, its also useful to stamp the main caps on one edge and a matched stamped number on the oil pan rail of the block, indicating which direction each main cap faces and its location during the original DIS-assembly process or first engine assembly to prevent potential screw-ups during later builds or refresh builds.
just make the stamped number clearly readable but not deeply stamped as you don,t want to induce potential stress risers that might weaken the connecting rods













yes I use both micrometers and snap gauges and cross check with plasti-gauge
and yes when you compare the crushed width of the plasti-gauge youll find it rarely falls as an exact match to the bar chart tape that is packaged with it so you can judge clearance based on crush width


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