Piston ring gap question

Piston ring gap question

Postby observer [OP] » September 14th 2020, 6:30am

I have the chance of setting the ring gap of a set of file-fit rings. My Haynes, Chilton, and factory service manuals all call for gap of 0.01-0.02" for the 1st (top) compression ring.

Has anyone ever set the gap at the lower end of 0.01" and ran into problems?

Assuming thermal expansion coefficient of steel of 1.3x10^(-5)/C, 0.01" ring gap, and piston diameter of 4", these translate to a max allowed temperature difference of 60C between the cylinder wall and the ring (before the gap closes).

60C does not seem a lot to me, but then I am no expert in engine thermal models. Not sure if 60C will be sufficient.

Thanks for any input!!
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Re: Piston ring gap question

Postby chevymaher » September 14th 2020, 11:12am

My engine builder did on my boats 3 liter. It got hot rings end butted and destroyed 2 cylinders. Best go the loose end and clock the ring butts 90 degrees on the piston.
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Re: Piston ring gap question

Postby K-TRON » September 18th 2020, 4:13am

I rebuild a lot of air cooled engines, and 0.010" is pretty tight for an upper compression ring. Water cooled engines run substantially cooler than an air cooled engine so I see no reason why 0.010" end gap would cause problems. You can always just go to a happy medium and set them at 0.015". The most ring related problems are from improperly bored/honed cylinders, dirty ring grooves or the wrong types of rings used.

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Re: Piston ring gap question

Postby gman » September 21st 2020, 6:51am

Never rebuilt an engine, but using the coefficient of expansion of steel is probably not a good idea, as that is probably applied to something that is 100% steel.
Lot's of things can be called "steel", and unless you know the exact chemical makeup of the rings, it's probably not going to be accurate.
Unless you can verify that the rings are made of 100% steel, I wouldn't use the coefficient of expansion.
Many things can be considered steel, and all of them have different makeups as far as composition goes.

You could use it to approximate, which it seems like you're doing.
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