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Thread: Oversized pistons and ring end gap

  1. #1
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    Oversized pistons and ring end gap

    I am in the process of tearing down a F series engine and have a few questions.

    First according to the specs, the bore is 3.125" and when I measure the bore it is indeed 3.125" and there is practically no ridge. Should come out with a little honing. But something doesn't quite pan out. The pistons all have 0.020 stamped into them. I assume that 0.020 means that the the bore and pistons are 0.020" oversized. Shouldn't the bore and pistons measure close to 3.145"?

    Second the end gap on the old compression rings is HUGE. Like 0.030" or bigger huge. And, yes, I used the piston to push it down a little to make sure it was square. Isn't that way too much? The manual says the end gap should be 0,008". What size piston rings should I get? I guess I could try the 0.020 oversized and hope the end gap can be ground to fit with a Dremel.

    Third, what is a good source for rings? I found them at JC Whitney. And the gasket set is available from OReillys.

  2. #2
    Super Moderator bmorgil's Avatar
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    Hmmm... I have to question your bore gauge. The .020 stamped on the piston does indeed mean it is bored .020 over. Since the ring gap on the top compression ring should be closer to 10 than .030, it almost sounds like you have standard rings on an oversize bore or, a very bad ring fitting job. If you can feel any ridge at all I think you may have more wear than you think, causing the gap to open up. As it has been written before the fit of rings and pistons to the bore is critical for a good running, long running engine. When a motor is down this far, fitting the pistons to the bore and the rings accordingly is the way to go, even if it means new parts. I think you might be better off taking it to the machine shop and letting him run his bore micrometers over it. He will be able to tell you what you need to do for a great long life ring seal. It sounds like .030 over would bring it back to new.

    The pistons are all the same. They come from Silvolite. The Rings are usually Hastings. For gaskets try to stick to the Fel-Pro brand. KW is a great source for the engine parts. You will want to substitute Fel-Pro for the Head gasket and Valve and oil pan. Some gaskets you cant get from Fel-Pro. I would use KW for those and the hard parts. They stand behind what they sell. If you don't like what you get you just send it back.
    Last edited by bmorgil; 11-03-2020 at 09:41 AM.

  3. #3
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    Thanks. There really isn't a ridge around the cylinder. It's more carbon build up than ridge. I'm going with a set of 0.020 oversize rings and see if the have a decent end gap.

  4. #4
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    The end gap on the rings was greater than I thought. The rings that came out had an end gap of 0.11" and the oil rings would fall out of the cylinder. It all adds up to zero or very little compression. And if it started, it would smoke like a freight train.

  5. #5
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    Well, I received the 0.030 over rings today and the manufacturer recommended a MINIMUM (yes, they used all caps) of 0.004" per inch of piston diameter. When I received the rings, they would touch ends and jam in the bore (interference ring gap). So using a dremel I took off a small amount, it doesn't take much, and filed the rest of the rings to 0.014" end gap. That ought to be good to go.

    Tomorrow, I install the pistons, oil pan, head and all the accessories.

  6. #6
    Super Moderator bmorgil's Avatar
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    It looks like your "in the book" Patriot. The service manual back in the 50's say's that a .020 over bore piston can be used in up to a .029 bore. The allowable ring gap for the little engine WITH a .020 over bore is as tight as .007" to a whopping .017" So it sounds like you have a .020 over bore.

    You are correct in using the ring manufacturers recommendations. On things like rings, the materials have changed from the original cast iron/steel designs. Out of curiosity, in what country were those PC rings manufactured?

  7. #7
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    They were Perfect Circle rings. I don't know what material they are made of.

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