Technical Piston Ring gaps /

Working on an Oldsmobile 350.
Machined 0.030 over, new forged pistons and Hastings moly rings.
The rings are supposed to not need any filing but I check gap anyway. bore is 4.087 top ring factor is .004 to .0045 do the math and that’s 0.0164 to 0.0184
Set top rings in and all 8 are at 0.020 on the money,,, humm

Second ring factor is .0045 to .005
Do that math and that’s 0.0184 to 0.020.
Set second rings in and 6 of them are 0.016 and 2 of them are 0.034

Wtf is going on?
I’ve never ever seen this, and don’t feel right putting it together.
Shitty quality control? I’m being too picky?
What would you guys do?

before you go asking,,
Yes I swapped rings around and the gap follows the rings. Yes is used a piston to set ring square an inch down the bore before checking.

 

Conventional wisdom is second ring gap bigger by .004-.006" than the top ring gap....this allows pressure that gets past the top ring to not get bottled up between top and second ring and cause ring sealing and flutter issues.
The fact a couple of the seconds are bigger than that is not any real worry....the second "compression" ring isn't really a compression ring, and if called on to carry the load because of a weak top ring the second ring will quickly fail.
Second "compression" ring is actually the final oil ring, scraping off excess oil and leaving just enough for the top ring to make a seal with. Case in point is the popularity of the second ring being Napier Scraper type on most modern ring sets.
The oil expander and rail rings are just the bulk oil control rings, so the second ring isn't overwhelmed with too much oil.

If you're not going to return the rings and see if you can't get a better matched set, I would file fit those .016" gap seconds out to .026" and let it fly....you'll never notice the additional .008" on the odd two that are at .034".

Rings gaps never were, and never are, the main leak point of a ring set.....ring groove finish in the piston is a lot more important, and the ring has to seal down against the ring land much more than it has to seal out against the cylinder wall.

With a piston-to-wall of about .0035", that means about .00175" of any ring gap is all the gap actually exposed to combustion pressure ....the thickness of 1/2 the piston-to-wall clearance. And that is cold clearance, hot from heat expansion and gap closes up even more.
You don't get much leakage through a not even .002" deep x .026" gap.

 

Right Eric,
The “no file” set top rings are too big a gap and the “no file” second ring gap is too small. Actually the gaps are closer if reversed.

 

I would exchange for a file fit set and have your way with them, you will feel better.

 

that's odd....I'd think about getting them replaced, but then again, I'd also run it like that, and it would work fine.

 

Most ALL..."ready to run" rings sets WILL be oversized. No matter the manufacturer.
Why...because of that little word..."litigation". If the rings are exactly... .004" per inch of bore...what are you doing for induction ?
That .004"/in is fine for a carburetor in a non-racing environment.
BUT, if you add a supercharger or turbocharger, or nitrous...that ring gap NEEDS to get larger.
BUT you bought ready to run rings...right, so you run the rings as received and then put a 300hp shot of nitrous oxide to the engine...then...the ring end gaps butt at 7500rpm, there goes the ring lands, then...there goes the engine..!
So, now you feel like suing someone...after all...it wasn't YOUR fault.

Now you might better understand why you're ready to run gaps are slightly large..!

If you were truly into horsepower, you'ld be buying an "oversized" ring set and setting the gaps yourself with the thought in mind of how the engine will be built and run .

Like me... I ordered a set of "light" pistons for an engine that I'm building. I FULLY understand, that I actually received "lightweight" pistons, and CANNOT run a blower or nitrous..!

Make sense ?

Mike

 

You guys might remember this
https://www.jalopyjournal.com/forum/threads/cam-1-tooth-off-olds.1223412/


Same engine going back together.
Last guy in there did him wrong.
I’m going to do the best I can for him which should be pretty good.

My machinist ordered the pistons and rings.
Either I forgot to specifically tell him “file to fit” or he forgot I specifically told him. Or perhaps he couldn’t get them. IDK but I don’t think it’s too much to ask they be all the same.

 

I've noticed that my filing technique can't quite match the precision of (most) pre fit ring sets. So, whenever possible, I use the normal rings....