I searched, but couldn't find anything on this.

I recently purchased a B17 block, it had already been sent to the machine shop to get hot tanked, bored and honed before I bought it. It was overbored .20, the block also came with a set of new ITR pistons that are .20 as well. The pistons are actually stamped with a .20 on the top. My questions is, I'm thinking of going with P30's instead, to get a higher compression, but am wondering if I purchased 81.25 mm pistons what would have to be done?....the obvious answer to me is it would have to be bored again right? I'm confused as to what .20 means, would this be the same thing as a 81.25mm piston?? as I think OEM pistons only come in 81 and 81.25 mills.

Dang, how'd this question get this long?....any help would be appreciated. Thanks!

 

.20 over actually means .020" over which is .5mm
.010" = .25mm
.020" = .5mm
.040 = 1mm

So you would need to get at the least 81.5mm P30.

 

Thanks for clearing this up man!

Are OEM P30's in 81.50 available? I thought my only options would be 81 and 81.25mm. I'm sure I'd be able to find aftermarket 81.5's, but I'd rather go with OG Honda pistons.

 

I don't think OEM pistons are available .020" or .5mm over, you should look into RS Machine though they got 81.5 and even 82mm pistons.