K100 master cylinder bore diameters.

[Tony Smith]

I am about to fit a K100 front end to my R65. i have the choice of either Brembo twin piston calipers as fitted to the early model K100's or the later model 4 spot calipers - either will fit the K100 forks and discs unmodified.

So, does anyone know the master cylinder bore diameters used in the early and late model K100s? My K100 manual helpfully doesn't tell me and my stock of complete K100RSs are currently stored at a friend's place 100km away.

I have a decision tree to follow once I know the stock bore sizes:-

1/. Pick 2 or 4 piston calipers (I'm leaning towards the twin piston ones as they are more than enough brakes for the R65, even with the performance enhancing modifications that are about to happen to it.

2/. Identify the nearest suitable diameter (probably a couple of mm smaller than stock given how wooden K100 brakes are)) Airhead master cylinder

3/. Decide whether to buy a new master cylinder or simply buy a rebuild kit and bore/sleeve one of my existing master cylinders to suit.
 
On that note, if anyone has a surplus/leaking/seized square master cylinder, I am interested in buying it from you.
 

[tunnelrider]

Hi Tony, a couple of days ago I ordered a master cyl. rebuild kit. I found the cylinder bore dia. stamped on the outside, underneath the master cylinder.

[Tony Smith]

Hi Tony, a couple of days ago I ordered a master cyl. rebuild kit. I found the cylinder bore dia. stamped on the outside, underneath the master cylinder.

Yep, know that. Pity the nearest K100 is a hundred kilometers away.

[mrclubike]

Looks like a 13mm

[mrclubike]

Late ones used a 20mm

[Tony Smith]

Agree, further research on my part this morning says that the twin piston Brembo models used a 13mm Master cylinder.
 
I'm pretty sure that the 20mm size is only relevant to ABS equipped bikes - I've thought of a way to verify that when I get home - my R1150GSA has 4-spot Brembos and no ABS, I'll check the bore size on that - although I am just about committed to running the 2-spot calipers as I think anything else would smack of severe overkill.

[Burt]

A late reply but yes the K75/100 2 valve engines all use a 13 mm bore, including ABS I models.  It is "stamped" on the upper surface.