Also can someone tell me whats the best way to measure stackheight for a headset thats already installed?
Base of crown race to top of locknut, then subtract out any spacers other than a thin one between the adjusting and locknuts + the headtube length. Alot of crown races are pretty generic for the cup and ball types so Sheldon is probably right with latitude for some oddball exceptions. I duo what the dimension for the french crown race is but ISO is 26.4 and JIS is 27.0. ISO cups are 30.2mm and JIS are 30.0.