and whats the advantages of going with 81.5mm instead of 81mm?

 

They are forged, with 81.5mm you get a tad more torque and compression because you're increasing your displacement a little bit....

 

They are forged. Over boring will also give you more compression as will as more c.c.s. Endyn

 

the .5 isnt really for more compression, its more for ensuring smooth and properly honed cylinder walls.

 

Yes, Larry offers them in that size or any aftermarket 81.5mm piston for that matter because you should be boring the block when you go through a rebuild. If you freshen a block well that's another story. But a result of increasing the bore with everything else the same is a slight increase in compression ratio. A 81mm x 87.2mm stroke at 11:1 has a chamber volume of 44.93cc and a cylinder volume of 449.3 when you bore to 81.5mm the compression ratio will go to 11.12. Not much but a little.