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Question about piston ring sizes

General engine tech -- Drag Racing to Circle Track

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bobalattie
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Question about piston ring sizes

Post by bobalattie » Mon Dec 02, 2019 9:49 pm

I’m building a 4.6 Ford that’s .020 over. The new pistons are 3.572, I was looking for rings and some are made 3.571 and some are 3.572. Why the .001 difference and is there really a difference? Would the 3.571 be less drag instead of the 3.572?

Or is this just a difference in branding and marketing?

Thank you for helping me to learn and understand

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modok
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Re: Question about piston ring sizes

Post by modok » Mon Dec 02, 2019 10:26 pm

Ring size is the size of the bore the rings are intended for. If the bore is .001 bigger than the intended bore size, gaps will be .003 larger, usually not a problem.

Piston should be smaller than the hole, unless it's a Subaru. :P
So, I don't know why the pistons would be the same size as the rings. maybe you mean... that is the size it says on the piston BOX, the intended bore size for the pistons. Which is generally a minimum.

s far as .001" difference is some specification, well, that's not outside the realm of possibility that nobody is sure what the official ford nominal size is exactly.
They have done that to us a dozen times before already, perhaps just continuing the proud FORD tradition of wide tolerances and selective assembly.
Very common to find the actual bore size .001-.002 larger than nominal, but rarely smaller. that's how it goes. Generally a +/- .0005, and if not specified that way then in this case the sizes in question are considered the low limit.

ProPower engines
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Re: Question about piston ring sizes

Post by ProPower engines » Mon Dec 02, 2019 11:14 pm

bobalattie wrote:
Mon Dec 02, 2019 9:49 pm
I’m building a 4.6 Ford that’s .020 over. The new pistons are 3.572, I was looking for rings and some are made 3.571 and some are 3.572. Why the .001 difference and is there really a difference? Would the 3.571 be less drag instead of the 3.572?

Or is this just a difference in branding and marketing?

Thank you for helping me to learn and understand
In a case of doing a build for more modern engines the stock replacement rings can and do vary greatly when compared
to a factory ring. the current method of machining rings from tube stock that uses a thinner wall thickness is also more
common now then before. Take a GM sm blk as an example. They used 2 different radial groove depths on the oil rings
on the 85-up engines with 1 piece rear seals. But now a stock ring set from several manufactures/sellers use a much
narrower radial depth rings. When comparing to a stock radial depth the replacements can be as much as a .100" on
the narrower side. The ring makers claim it does no affect the sealing of the ring but was also told by the same companies tech guys its for cost of manufacturing savings.

Best bet is to call your favorite ring supplier and get a file fit set to get them perfectly fit to the pistons and finish of the cylinders but to get the best ring gaps for your applications use
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