So I just checked again, Wiseco part # 40219M06600 is a 66mm piston for EX300 (will fit Versy-X 300, same engine) replace 6600 with either 6400 or 6200 for 64 & 62mm versions, all have 16mm wrist pin, as do the 250's I believe.
So 62mm would up compression from stock 10.6:1 to 12.5:1, the others are adjusted to keep cmpresssion at 12.5:1 regardless of bore, less material above the top ring.
JE list a full set of pistons for the 250, so would need the inlet pockets opened a little for the larger intake valve, but that is all I think. The stock JE 62mm high comp piston is 134g, the 66mm FSR slipper piston is 137g, say minus 1g for opening the valve cutout a little, so say 136g, which is not bad.
I need to confirm the stock 300 piston weight ! Wiseco unfortunately don't publish there piston weights, but I have seen mention in someones 282cc big bore of them being 152g, which is crazy.
Any info on 300 piston weights is appreciated. Franken Big Bore on the way
