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Piston Ring Gap

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Old Nov 16, 2025 | 08:46 PM
  #1  
belacyrf's Avatar
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Default Piston Ring Gap

Question for those who may know. Assembling my engine and decided to measure the ring gap. I had the rings gapped by the machine shop, but in measuring today, they were all well past spec.
Per the Wiseco spec paper for a boosted miata, The first ring is. 0050 and the second ring should be. 0055.

My rings were all at least. 0060, mostly .0070 to .0090, and one even larger than .0090. And no rhyme or reason... some times the 1st ring had a larger gap, sometimes smaller.

My question.. Am I doing it wrong or misunderstanding? Or did my machine ship screw up?

UPDATE: So yes I'm doing it wrong. I'll leave this here for others. Who (like myself) struggle reading
Ring Gap is measured by multiplying bore size by the standard ring gap.
So in my case for Wiseco Rings:
84MM Bore = 3.307 inches (84 / 25.4).
Top Ring Gap for "Boosted Engine" = .0050 per bore inch. So mine should be .0165 (3.307*.0050)
Second Ring Gap for "Boosted Engine" = .0055 per bore inch. My second ring gap should be .0181 (3.307*.0055)

So my rings, definitely too small. <whew>. Back to the machine shop I go.

Last edited by belacyrf; Nov 17, 2025 at 10:53 AM.
Old Nov 18, 2025 | 04:24 PM
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FWIW, getting a ring grinding tool and gapping your own rings isn't the hardest trick in the book. Did you specifically tell them you wanted .005" gaps? Any machinist being asked that should've stopped you right there. That's tight even for a naturally aspirated econobox motor. Also, if the machine shop really left that much of a delta between different rings, and were that far off of your target gaps, I wouldn't be stoked. There must have been a misunderstanding somewhere along the line. A boosted motor with rings gapped between .007" and .009" isn't going to last very long.
Old Nov 18, 2025 | 09:49 PM
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curly's Avatar
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They probably weren't touched out of the box.

I think I shot for .018 top ring, .020 bottom ring with 84mm pistons. Reason being most here said the stated specs were a touch on the tight side. Bottom ring is much softer and will grind easier, so don't be surprised if 1 or 2 end up a little on the big side. In my reading, recommendations ranged from Wiseco's gap of .016 top, .018 bottom, to .022ish top, .026ish bottom.
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