Front Master Cylinder Replacement

[Joshfire]

Wanna know the sizes of the bore and thread size so I can get a replacement. I heard 14mm with a 5/8 is the oem size but not certain. On my original MC it says 5/8 on the bottom. My bike is a 1975 Yamaha xs650

 

[5twins]

5/8" was the original bore size for your '75 model. 14mm is the bore size for the '77 and later models. Your 5/8" original bore size converts to just under 16mm. It's said that the later slightly smaller 14mm bore MC works well on the early models like yours. But I guess it all depends on how original you want the thing to look. The later MC has a more rectangular reservoir while your original has a round one.