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Piston ring end gap (filing piston ring ends)

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steppinthrax

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This is kind of an advanced topic.....

I'm assembling a Kohler tractor engine. I've done pretty much everything as far as getting the cylinders honed and valves lapped. I've ordered new piston rings (STD).

The piston rings (OEM) are the "filing type". They come from the factory .003 and you must file then to fit the bore. The gap (per engine spec) is .010" along the smallest bore (if taper exist). The question is the oil control rings are a set of 3 rings (two rail rings and a corrugated spring ring).

Do you ever need to file the oil control rings? I've looked everywhere and there is no mention about this. Only literature about filing the two top rings (compression). I understand that you should NEVER file the corrugated spring ring.
 
The compression rings are the ones you need to worry about.
The oil rings should have a gap from the factory, just need to orient so they dont all line up durring assembly.

Question, you got the cyliner honed how much bigger? Are STD rings going to be big enough? Didnt need bigger pistions? So hone with no bore? What was the issue with the engine?
 
jaha2000 said:
The compression rings are the ones you need to worry about.
The oil rings should have a gap from the factory, just need to orient so they dont all line up durring assembly.

Question, you got the cyliner honed how much bigger? Are STD rings going to be big enough? Didnt need bigger pistions? So hone with no bore? What was the issue with the engine?
Click to expand...

Just de-glazed the cylinder. I did some measurements on the cylinder and it was within spec. I decided to use std rings. The engine was from a previous owner. I got it with a broken conn rod. From what I can tell, there was low oil and one of the journals ground and heated up the conn rod bore. The rod snapped and tore up the piston skirt.

I carefully inspected the bore and there was no damage/score marks. The crank journal just had aluminum transfer. I put it in my lathe and simply used 400 girt with water and it cleaned up beautifully. The journals were also within spec.
 
10 or so years ago I did manufacturing engineering work at a company that built diesel engines. My memory is that we did not touch the oil control rings, we only did the top 2 sets. I could be wrong but am pretty sure on this. I can't think of any reason for you to do it differently. But I guess I can't see it hurting either. Hmm that's not much of an answer, sorry.

I'll mention too that you should check the bore diameter in multiple spots and set the rings for the tightest spot you measure. If it turns out to be slightly tapered or barrel or hourglass shaped you don't want to have the rings set for the largest diameter and end up having the gap go to 0 at the smallest diameter and start causing the rings to scrape the cylinder wall. Sounds like you didn't take much material off so you're probably OK but I'd still be sure.

The oil control rings tend to be so flimsy in comparison to the others, perhaps that is why no one bothers with them?
 
tracerbullet said:
10 or so years ago I did manufacturing engineering work at a company that built diesel engines. My memory is that we did not touch the oil control rings, we only did the top 2 sets. I could be wrong but am pretty sure on this. I can't think of any reason for you to do it differently. But I guess I can't see it hurting either. Hmm that's not much of an answer, sorry.

I'll mention too that you should check the bore diameter in multiple spots and set the rings for the tightest spot you measure. If it turns out to be slightly tapered or barrel or hourglass shaped you don't want to have the rings set for the largest diameter and end up having the gap go to 0 at the smallest diameter and start causing the rings to scrape the cylinder wall. Sounds like you didn't take much material off so you're probably OK but I'd still be sure.

The oil control rings tend to be so flimsy in comparison to the others, perhaps that is why no one bothers with them?
Click to expand...

Ok thanks.

The engine manufactures Kohler no longer provides support and refers people to general engine shops, most of them don't get down to this level for such a small engine.
 
I'm certain that if you put the oil control rails into the bore and square them up with the piston that you'll find that the gaps will far exceed the specs for the compression rings. The two rails and the expander are an assembly for "oil control", not elimination. The excess oil is scraped off the bore. Some oil needs to get through to lubricate the compression rings.
 
The top two rings are all you need to adjust for file-fit rings, the oil rings run as they come. A note for future reference: Ring gap is generally calculated at .004" per inch of bore. I don't know the bore on that engine but .010" would be recommended for a 2 1/2" hole.
 
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