ok i am looking at rebuilding my motor and came across a good pistons/rods combo from a member on another site. The only problem are the pistons are 81mm instead of 81.5. are there any problems with running these and how would you go about machine the block to fit them? im not resleeving because my goal is 400hp

 

if your block is 81.5, you need 81.5 pistons

 

well i know stock bore is 81 mm, but when rebuilding you usually bore over .5 to clean up the cylinder walls. its a b series btw

 

You don't have to overbore 0,5mm but most people do to ensure a clean, round sleeve. Having said that, i'm running an 81mm built bottom end. If you run the block to a machine shop and have them check taper and out-of-round you should know whether or not you need to overbore. If those results come back positive, buy the 81mm pistons...otherwise, overbore and find another deal elsewhere