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!

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