R65lsMaster cylinder diameter

[balibeemer]

A fellow airhead has managed to get hold of an R 65 LS produced Nov. '81. Now, heres the question:
We are going to replace all the handlebar stuff including the hydraulic front brake master cylinder. To order the parts from Motobins in England, we need to know the diameter of the master cylinder( 12, 14, 15 or 16 mm). Does anyone know?
and YES, now there are 2 of us in Bali, so the airhead population grew by 100%

[Mike V]

Balibeemer,
Check on the bottom of the master cylinder case or housing, the size should be stamped on the housing. I believe there were two sizes that year - 15mm and 16mm. Either one will work.  The 16mm will give you slightly better lever feel and the 15mm will give you slightly better braking. My 81 R65 came with a 16mm M/C and I replaced it with a 15mm. The differences are very slight.

http://i428.photobucket.com/albums/qq8/gruntyman66/81%20R650%20Restoration/Front%20Brakes%205-09/81R65Restoration016.jpg

Mike V. / San Diego

[drewboid]

The 12mm master cylinder was correct for the single caliper R65 so that would not be correct for you. 14mm was used on some of the older /6's with an under tank MC.
I added a second disk and used the 12mm for a while and then changed to a 15mm MC. More braking for less movement but required slightly more effort. I doubt that you would find much difference between the 15 and 16mm masters.
The size should be indicated on the bottom of the unit. I think the 81's used a round reservoir, my 84 has a rectangular one.

[Mike V]

Drewboid brings up some good points. Maybe I should clarify my earlier post; I'm assuming you have dual disc/caliper applications. This year (81) seems to be a transition year for fitment regarding circular and rectangular M/C's. Mine is a rectangular and my experience is only with rectangular and dual disc/caliper applications. Vintage Brake has a ratio chart that is interesting to better understand ratio's and their characteristics on braking operations. Work through the math and you can determine which way you may want to go (15mm or 16mm). As I mentioned before, the difference between 15mm and 16mm is slight.

http://www.vintagebrake.com/mastercylinder.htm

I physically measured my caliper pistons to be 36mm. With the installation of the 15mm M/C my lever feel seems to be acceptable with more than adequate pressure to the calipers for braking quality. Hope this helps.

Mike V. / San Diego

[Justin B.]

1981 and later should have the rectangular MC...

[Ed Miller]

Doesn't a Nov. 1981 build make it an '82 model?  It's probably a single disk, but Balibeemer can figure that out easily.
  

[balibeemer]

Gentlemen - Thanks for all the info  - Its a twin disc so we'll probably go for the 15 mm and better braking.