blk~2000~z28

Hello all, Just got a 1983 Yamaha XS400 last week. It took some TLC but it now goes down the road, however the front master cylinder assembly was not working and needs replaced. After searching I think this (Part #08-0255) at this site www.mikesxs.net will work. How do I determine what mm bore I have, to make sure? thanks, Greg