Oversized Piston Question

I plan on putting oem honda oversized pistons in my b17a when i rebuild it. I was just wondering how big oversized pistons are. Some people tell me “0.25mm” others tell me “0.025inches” so which is it?


As for as I know they are usually oversized 0.5mm. Whatever it is its got to be metric.

well, everywhere and everyone i talk to just says “25 over” Thats what it says on www.jdmhondaparts.com as well. And it seems to me that .25mm would just be too small, i mean a quarter of a mm is pretty tiny. .025" however is .0635mm, which sounds to me like a more reasonable size to bore over.

but i’m really not sure. Either way, i know its not .5, cause the place that sells the pistons says they are “25 over”.

anyone else?

Ok after a bit of brain power I’ve decided its because of the whole radius vs. diameter thing. A quick reference onto Endyn’s site confirms he says 81.0mm and 81.5mm as options.

Ah yes!!! I solved it!!!

diameter vs radius? so that means when they say “.25” over, they mean .25mm larger radius? therefore .5mm larger dia?

thats really odd. I never would have even thought they would measure it that way. I had just thought it meant how much larger the dia was!

thanks for the help!

Honda pistons are 0.025" over. Most other aftermarket pistons will come in 0.5mm overbore and if you convert that to inches would be somewhere very close to 0.020". Of course there are also 82mm, 84mm, 84.5mm, etc size pistons. HTHs.

ok, a different answer now. so, honda oversized pistons are 0.025" and NOT 0.5mm I know the two are close in size, but i’d like to know for sure.

Yes, when they say 25 over they mean 0.025" over 81mm. Not sure what 0.025" is in mm’s. Aftermarket Wiseco pistons, for example, come in the 81.5mm size which is almost equivalent to 0.020" over 81mm. Hope that clears it up a bit more for you Colin. Good luck with your motor buildup.

.025" = .0635mm

so, thats .0135mm difference between .025" and .5mm oversized.

leo, importstop doesn’t happen to sell oem honda pistons do ya? Cause i’m most likely gonna be using oversized p30’s or jdm itr’s.


Yes just to confirm. With a calculator, 0.025" pistons are 0.0135mm larger than 0.5mm pistons.

Right now, we only have access to CTR and US ITR oversized pistons. Otherwise, we wil be carrying JE, Probe, and Arias next week if you’re interested in aftermarket forged pistons. Let me know.

My understanding is .25 over means 0.25mm over the standard 81mm bore. Hence you hear people talking about 81.25mm bores. 0.25 over is what the factory recommends if your cylinders need re-honing.

Please feel free to correct me if I’m wrong here.

Lyonel is correct.

I stand corrected.

ok. so, now for sure, oversized honda pistons will be 81.25mm ??

man, that sucks, i was really hoping to gain at least a small amount of displacement from going oversized.

are there any larger dia pistons i can use to bore over even more, that won’t be too expensive? Nothing like 84mm or anything where i’d need new sleeves.


There are 82mm pistons out there also.

82mm, sounds nice. but i’m guessing they would be more expensive than $350 for pistons and rings ?

25 over bore gives you 24.9cc more. Thats all. :frowning:


steve, if .25 over means you should have an 81.25mm bore, then you only increase displacement by 10.3cc!!! which really sucks.

before i was under the impression that 25 over meant .025" over. Which would then increase displacement by 26.41cc, which still not much, but its gotta count for something.

i’m wondering where you got 24.9cc, what numbers did you use?

Some one told me that once upon a time. I thought they knew what they were talking about. Ooops.