Basically there were 2 pistons used in the B40 standard ratio 7:1 and higher comp version 8.75:1,the higher one now seems to be favoured by the more experienced(and more aggressive riders),i still have about half a dozen of these all 1/2mm (+020) oversize,can deliver to the bonanza if anybody wants one,the low comp piston measures 1.608" from centre of pin to top,high comp piston measures 1.801" for identification. Pistons i have are all genuine BSA