Thank you guys for your replies , having asked around the trade I am going with the center of the pin. My problem is that not all suppliers quote the crown height and the ones that do don’t say where they take the measurement from. Some will just ask
what machine is it for, In my case this is no good as I am looking for a pair of pistons to fit an Adler MB250 twin that I am rebuilding, It will have Kawasaki KX125 motocross conn rods. All good fun While in lockdown.