Another rings end gap question

Discussion in 'Rotating Assemblies' started by PrefixAM, Nov 27, 2018.

  1. PrefixAM

    PrefixAM Member


    I'm rebuilding my 4.3L Vortec from 2003 Chevy Astro Cargo Van. When I bought it a year ago it had ~26.000 miles. The engine was knocking and I couldn't get rid of that annoying sound - tried to clean up lifters through the push roads and by "seafoaming". It didn't work out, so I just decided to pull the engine out and rebuild it, because I also wasn't sure if that was the accurate mileage.

    Anyway, I disassembled the engine - it looked perfect to me. I sent it to the machine shop, they also said that it's in very good shape, they didn't bore it, just honed. So it appears that the mileage is correct.

    Now closer to the subject - time to rebuild the engine. Per service manual, the cylinder bore size is 4.0007"-4.0017". Most sources tell that in order to calculate top ring gap you have to multiply 0.004 per inch of bore for street applications. Which gives that well known 0.016" gap (either way if I use 4.000" or 4.0017" as a base bore, the formula gives me 0.016" - 0.0160068" gap).

    I bought first set of rings - Clevite MAHLE 41786, standard, since machine shop guy said bores are standard. Inserted a top ring, slightly pushed it with the piston, measured the gap and it was ~0.026". "Way to much" - I thought, but measured few other rings and put them in different cylinders - results were pretty much the same.

    I though that probably machine shop guys did something wrong. So I bought a set of micrometers and telescopic gauges. Measured bores - 4.0022". That still keeps us in the range of 0.016" (0.0160088" to be exact). Then I thought, well, perhaps bores out of round. So I bought a bore gauge and checked all bores - all perfectly rounded (variation in 0.0005" which corresponds to the production Out-Of-Round specs). Then I thought - maybe it's just a bad rings set. So I returned that Clevite Mahle rings set.

    I bought another one - Sealed Power E920K (standard) set and received it today. Checked the gap, it was 0.027" - 0.029" (depends on how deep you push a ring with the piston plus how perfectly perpendicular you do this).

    So it looks to me that the manufacturers are cheating by making such a large gap so the rings fit both cast AND hypereutectic pistons. In my case, I'm reusing my stock standard pistons.

    Now here is the question. What would you do - return these rings and buy "file to fit" rings and make 0.016" gap, or install these 0.028" rings?

    P.S. I found TONS of priceless info on this forum, thanks.
    1_ring.jpg 2_telescopic_gauge.jpg 3_micrometer_reference.jpg 4_micrometer_telescopic_gauge.jpg
  2. chromebumpers

    chromebumpers solid fixture here in the forum Staff Member

    Welcome Prefix, I can’t help you for this particular question but somebody will be by very shortly that will help you
  3. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    If you want to set your own piston ring gap to your specs you must buy a File to fit piston ring set.
    Typically. 005" oversized.

    Total Seal piston rings are the best source to buy.
    Costs more Yes.
  4. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Speed Pro is another good source.
  5. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    the truth here is that if the ring end gap is anywhere in the .016-.030 the rings will work out fine,short answer you can use the rings you have they will work just fine,
    if properly installed with the mildly larger than ideal gap,and they may even prevent engine damage if you mistakenly over heat the engine

    the piston ring manufacturer's know from testing that compression and oil control,
    emissions and all other test results tend to show ring gaps under about .040,
    have nearly zero effect on how the engine runs, how much oil it uses or its ability to pass emissions,
    that .016 is the IDEAL end gap on a daily driver engine , remember as the pistons heat up the rings expand,
    and the end gap narrows significantly. a .016 end gap should result in having the ring ends almost,
    but not touch under normal operating conditions, almost all ring manufactures strongly suggest a bit larger end gap,
    if you use any power booster , as that tends to noticeably increase operational heat and result in tighter end gaps.


    Last edited: Nov 29, 2018
  6. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    It's a Crappy engine a GM 4.3 compared to a Buick 3.8 L V6.

    A Pontiac V8 and BBC are 1 million times better.
  7. PrefixAM

    PrefixAM Member

    Thank you for your feedback guys. Thinking about that gap issue, I made the following conclusions. While production specifies that bore size might vary from 4.0007 to 4.0017, the manufacturers clearly state that they produce rings for 4.0".

    Mine bores are slightly larger than production, but technically speaking, they 0.0022 larger than the base 4.0" size. Multiplying that by 3.14, and we get 0.007 difference between rings for 4.0" bore and 4.0022". Meaning that if these rings would've been installed in the 4.0" bore (as the manufacturers say - base bore is 4.0"), then the rings would have a gap 0.0027"-0.007" = 0.0020" (for the first set it would be even smaller - 0.0026"-0.007"=0.0019").

    With that being said, my conclusion is that the best thing to do in such situation is to go with Service 1 procedure, i.e. +0.010 for base size.

    I tried to find more information about what is "too large" for the ring end gap, but no clear answers from the manufacturers. Now thanks to you guys for your opinion on this issue. Most likely, I will install these rings.
  8. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    you have the option of ordering slightly over sized rings and carefully file fitting the end gaps,
    no mater what size rings or end gap , you use the rings will normally take 5-20 minutes or so to lap into and perfectly seat and fit the bore walls
    obviously the closer they match the application on start-up the more rapidly you could expect that to happen.
    remember a properly honed bore using deck plated helps the process a great deal.
    no one has mastered all the skills and going back over even rather common assembly skills and looking for tips on how to improve existing procedures seldom hurts.
    as time progresses, theres always on-going documented testing, and in many cases the old established way of doing things has proven to be less than ideal as newer and more detailed testing proves.
    I well remember the advice in the later 1960s to keep end gaps on upper piston rings in the .004-.005 per inch of bore diameter, and secondary compression rings , to a tighter .04 max per inch of bore diam, as they experience less heat related expansion, the gaps could be tighter, well testing over the last 40-50 years has proven that approach to be less than idea, a looser .005-.006 second compression ring end gap is now been rather conclusively proven to work a bit better as it tends to lower and trapped combustion pressure, that could reduce the top rings bore seal.
    Last edited: Dec 10, 2018
  9. PrefixAM

    PrefixAM Member

    could you please advise, does it make sense to order file-to-fit 4.0" rings, or I will need larger size? do they usually have no gap at all, or they also come with some sort of default 0.010" gap? I'm thinking what's the best option for mine 4.0022" bore, file-to-fit 4.0" or file-to-fit 4.005"?
  10. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    I generally try to get a .020-.022 end gap,on a 4"-4.030 bore, size,
    .016-.017 may be recommended, ring end gaps under .027 are ok,
    but if a newer engine over heats and the ring ends butt the rings expand, AND lock in the bore,
    and as a result, is frequently part of the top of the piston pulls off.
    Last edited: Nov 29, 2018
  11. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Not sure what year of Pontiac 400 V8 Block Grumpy was used.
    Found this on Chevy Talk.
    Early 400 Pontiac 1967-1974 rumored to be best.

    Taking no chances with my TA Pontiac 455 engine.
    Bit the Bullet & Ordered myself a BHJ Pontiac 389-455 Torque Plate.
  12. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    Welcome. Here is my 2 cents: your cylinder bores are in great shape, but not perfect. Therefore do not try to use fancy rings.
    Choose a moly coated cast iron ring set, such as Hastings. They will seat and seal instantly and last for the life of the engine.
    They are inexpensive also. And you set the gaps according to what the PISTON manufacturer recommends.
    And the oil expander rail gap ends point to bottom of piston. Like this: \/\/\/\/\ /\/\/\/\/
    It is your choice as to whether you use the standard (pre-gapped) set, or file fit. I would use the standard set unless you have
    experience file fitting rings. You can make things worse if you don't know what you are doing. Here's something from my notes:

    RECENT TESTING HAS CONFIRMED…..that pressure bypasses the top ring that gets trapped between the first and second compression rings and tends to significantly reduce the top ring seal, thus the current recommendation is for the second ring to have a slightly larger end gap to significantly reduce that and the pressure that leaks by the 2nd ring gap helps to keep the oil rings cleaner. Extensive testing in recent years shows that 1) the second ring gap needs to be LARGER because if significant cylinder pressure builds between the top and lower ring the upper ring seal is quickly lost.

    2) There is very little cylinder pressure lost thru the ring gaps in the thousandths of a second the rings are compressing the fuel/air mix, or during the power stroke, because most of the blow by is the result of less than effective ring to cylinder wall seal. 3) Ring gaps up to about .045 have very little effect on blow by or oil use.

    Good luck.
  13. PrefixAM

    PrefixAM Member

    Thanks again everyone. I ended up buying file-to-fit Mahle Motor Sport 4005MS-15 rings, they come as a set for V8, so it was good to have 2 more rings in case I f*k up some of them (which I did). On Saturday I gaped them as 0.019" (top compression rings) and 0.020" (second compression rings) and put with the pistons in the block yesterday. Without camshaft in place, it takes ~12ft.lbs to turn the crankshaft which is fine according to the information I saw on this site.

    @Loves302Chevy, too late, assembled with oil expander rail gap ends pointing top, hope it won't harm - based on the information I see it should not be critical (correct me if I'm wrong).

    file-to-fit_rings.jpg engine.jpg
  14. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    Actually, I don't know if there is a difference. And I have never seen anything about it on any ring install instructions.
    I forgot where I got that info, probably right here on Grumpys.:D
  15. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Not that critical Mike that oil ring detail.
  16. Indycars

    Indycars Administrator Staff Member

    With that ring filer you show in the picture above, it will file the ends such that they are NOT parallel
    when you put them in the cylinder to check the end gap. You will end up with a gap that's pie shaped.


    You have to move the ring away from one of the stop pins. The grinding surface of the wheel in not on the
    centerline, the middle of the stone is thou.


    Don't forget about cleaning up the burrs created by the grinding.

  17. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    thanks for re-posting some great quality pictures, and related info RICK!
  18. PrefixAM

    PrefixAM Member

    Thanks, that's exactly what I did. When you manually grind from 0.010" to 0.019" you have plenty of time to adjust your technique :)

    Another aspect that I can point out as a person who's done that for the first time - the bottom compression rings are grinding way faster. Because of that, when I did the first bottom ring expecting it to be around 0.0016" (based on the experience of grinding the top rings), it appeared to be 0.023".

    Yep, did that as well.
  19. Indycars

    Indycars Administrator Staff Member

    Sounds like you are on top of it then, nice!
  20. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    Answers On The Second Ring: The Science Of The Second Piston Ring


    Since the invention of the metal piston ring at the start of the industrial revolution (which, you could argue, finally made steam power practical), there’s been constant innovation and improvement in cylinder sealing technology for these seemingly simple parts. The ring package has three primary goals: Keep pressure confined to the combustion chamber on both the compression and power strokes, transfer heat from the piston to the cylinder walls where it can be removed via air or liquid cooling, and control lubrication to limit oil consumption and unwanted emissions.

    While it’s easy to look at the top ring or the oil ring at the bottom and intuitively understand their contribution to meeting these objectives, the second ring is more of a mystery. What’s it supposed to be doing, and why is it necessary? How do the materials used and the physical properties of the second ring affect performance? To answer these questions, we turned to Wiseco Senior Technical Account Manager Alan Stevenson.

    Under Pressure

    For our first question, we asked Stevenson whether the second ring had a role in containing compression or combustion gasses. “There was a time when bores were so bad in terms of surface finish, roundness, and so on, and ring materials were a lot worse, so that pistons used to have four rings; two for compression sealing, one for scraping oil, and one for pumping oil,” Stevenson explains. “The terminology hasn’t kept up with the technology. Referring to a contemporary second ring as a compression ring is a misnomer.”

    So what’s the contribution of a modern second ring to combustion chamber sealing? Per Stevenson, “Negligible. There have been SAE papers published that prove how enlarged second ring gaps actually increase top ring sealing and power. Combustion sealing is 100-percent the top ring’s job.”

    Note the subtle hook shape at the edge of the ring. This is a Napier-style second ring and that “hook” helps it pull oil off the cylinder wall as the piston slides down the cylinder bore.

    Combined with other piston features, the second ring’s role in this respect is to keep the pressure in the crevice space between it and the top ring as low as possible, giving any blow-by that makes it past the top compression ring a way to quickly escape to the crankcase.

    “An accumulator groove works in concert with larger second-ring gaps,” Stevenson explains. “In short, there will always be some combustion pressure leakage past the top ring due to secondary piston motion and cylinder crosshatch. Any pressure that makes it past the top ring tends to get trapped between the top and second ring, which then pressurizes the top ring from underneath which leads to ring flutter (especially at high RPM).”

    “The accumulator groove creates additional volume which decreases pressure. This is where Boyle’s law is applicable; volume and pressure have an inverse relationship, so increasing volume of the chamber decreases pressure. Coupling this with larger second-ring gaps provides a smoother transition of the trapped gas out of that space and reduces top ring flutter.”

    Because the second ring is specifically intended NOT to be a pressure seal, it’s often constructed quite differently from the top compression ring. “Many top rings have inside diameter bevels that cause them to twist opposite of the forces acting upon it in order to help keep it flat in the groove for better sealing,” Stevenson says. “Second rings have a bevel opposite to that, so they actually twist the wrong way to help sealing.”

    While the oil rings do the brunt of the work “pumping” oil away from the cylinder wall face, the second ring plays a vital role scraping it off the face of the cylinder.

    The Heat Is On

    Having established that the second ring is most definitely not there to provide compression or combustion sealing, let’s look at the second main objective of the ring package: transferring heat out of the piston and out to the cylinder walls, where it can be managed by the cooling system.

    It might seem like the relatively tiny amount of contact the rings make between the piston and the bore couldn’t possibly be a significant route for heat conduction, but it turns out to be the major provider. Per Stevenson, “There are many variables here, but the rings transfer about 70-percent of combustion heat from the piston to the cooling system.”

    The remaining 30-percent escapes via other routes, like radiation and convection cooling of the underside of the piston to the air inside the crankcase, conduction cooling through contact between the piston skirt and the cylinder bore, and heat carried away via oil splash from crankshaft windage. Some engines even employ oil squirters at the bottom of each cylinder bore that direct a spray of lubricant at the underside of the pistons specifically to aid in cooling.

    As rings continue to shrink in dimension to reduce friction, materials and manufacturing becomes far more critical. Carbon steel is the material of choice for most performance applications and offers robustness and durability far exceeding earlier, thicker rings.

    Other sources of heat transfer notwithstanding, the ring package handles most of the load when it comes to keeping the piston at an acceptable operating temperature. Stevenson further breaks down the previously mentioned 70-percent of total piston heat, “The top ring transfers 45-percent, the second ring 20-percent, and the oil ring 5-percent,” says Stevenson. While the second ring definitely plays its part in this critical task, it’s still not the ring’s primary reason for being there.

    Oil Control For The Win

    As it turns out, the second ring has a lot more to do with lubrication control than the “oil ring” beneath it. “The second ring is what scrapes the oil,” Stevenson explains. “The oil ring is what gathers it and pumps it away from the cylinder walls via oil return holes in the oil ring groove.”

    The second ring’s main function is to continuously remove excess oil from the bore — as the crank rotates, oil escaping from the pressure bearings on the rod big ends is constantly thrown up behind the piston, coating the walls of the bore. On the downstroke, the second ring and the oil ring work in concert to clear all but a tiny amount of oil and return it down the bore to the sump.

    “The top rings will always receive latent lubrication by oil trapped in the cross-hatch of the cylinder walls,” Stevenson says. It’s that microscopic texture on the bore that retains just enough oil to keep friction between the ring package and the cylinder wall to a minimum, while the second ring prevents too much oil from making it up past the top ring and into the combustion chamber.

    Thinner piston rings are far more likely to be damaged during installation than older-thicker rings. Using a tapered ring compressor should be considered an absolute necessity when building a late-model engine.

    Theory Into Practice

    Now that we understand each ring’s purpose in the package, we can see why different specific materials and ring cross sections are often used for the top and second rings. “The demands and intended function of the top and second rings are different, for sure, so the materials often are as well,” Stevenson continues.

    “The overall best top ring material is steel. Now, granted, some steels are better than others, but as rings get smaller and specific output increases, the demands on the top ring (which sees the most abuse) are highest.”

    Move down a groove on the piston, and the different job being performed places lower demands on the material being used. “Many second rings in racing engines are still cast iron or ductile iron. The second ring is not under enough stress and temperature to necessitate steel,” Stevenson explains.

    Setting the proper ring gap is paramount to achieving the desired engine operating characteristics. In any performance application, the second ring gap should be larger than that of the top ring to allow blow-by to escape and prevent ring flutter from upsetting the top ring’s seal

    The shape of the ring profile also has a significant effect on how efficiently it removes oil, as well as how much friction it introduces, and both the interior and exterior diameters have a role to play. “Bevels are on the inside diameter of the ring and dictate the direction the ring twists to aid in scraping,” Stevenson says. “Taper, Napier and steps are all variations of the outside diameter shape.”

    Viewed in cross-section, a beveled ring has one edge of the inside diameter cut at an angle, as Stevenson points out, this encourages the ring to dynamically twist in the groove as it moves down the bore and focus additional pressure on the outside corner — in order to more efficiently sweep excess oil away.

    As the piston moves down the bore, the second ring “scrapes” the oil from the cylinder wall, both providing lubrication and keeping oil from entering the combustion area of the engine.

    The goal with all these profiles is to concentrate contact into a narrow band to increase the efficiency of the scraping action. As the name implies, a tapered outer profile is narrower at the top than at the bottom, while a stepped ring profile has what looks like a notch in the cross-section, oriented toward the direction of travel on the downstroke.

    A Napier ring, named for the famed British D. Napier & Son engineering firm that originally developed the profile, is actually undercut at an angle or even hook-shaped on the outside diameter, further decreasing the contact area and providing space for scavenged oil to escape, away from the cylinder bore. “In order, the most efficient scraper is Napier, followed by step, followed by taper. Run a Napier if it’s available in your bore size and suits the groove in the pistons,” Stevenson concludes of second-ring face shapes.

    Gas ports are another way piston designers can manipulate how rings work. By allowing combustion pressure to access the backside of the top ring, they increase ring seal increasing power while reducing friction on the other three strokes.

    Application Dictates the Details

    What kind of a combination you are running will also influence the optimum choice for your ring package, including the second ring. “Thinner second rings are more prevalent in dry sump engines pulling gobs of pan vacuum,” Stevenson advises. “Naturally aspirated with no vacuum help, the ring should usually be 1.5mm or larger, while forced induction should err towards even-larger 1/16-inch rings.” Because crankcase vacuum helps ring seal across the board, it’s possible to get the desired results without working the second ring quite as hard.

    “Of course, this is all relative to bore size; you can almost think of it as a ratio of ring size to bore size,” Stevenson cautions. “A big boost four-cylinder engine will control oil just fine with a 1.2mm ring, while a 4.600-inch-bore big-block would be happier with a 1/16-inch ring. There are also substantial variables in crankcase efficiency when it comes to oil control. Modern engines with deep-skirted blocks, segmented oil pans, windage trays, and crank scraping/scavenging all have an effect on how much oil is thrown up into the cylinders. The more oil present, the harder the second ring’s job is.”

    As you can see, second ring design and engineering is a complex subject, but fortunately, the experts at Wiseco have the collective experience in all forms of high-performance engine builds to provide you with sound advice for your particular needs. While we can’t cover everything in a single tech article, we hope that what you’ve learned here will help you to better understand the “why” behind a ring package’s specifications, and take full advantage of the knowledge on tap from Wiseco’s staff when putting together your own combination.

    As with any other engine component, there are tons of different styles and ring materials that are suited to unique applications. Consulting an expert is always the best way to match rings to your engine build.

    ed. note: This article was provided by Wiseco Pistons, and we felt that the editorial merit was worth sharing it with you.

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