The International CBX Owners Association

I'd like to reduce the hand pressure required for braking(an easy two finger operation would be nice). My mechanic says I switched from a one pot to a two pot I maybe should have switched master cylinders at the same time. The cylinders may have had different diameters, he thought maybe the oem was possibly 5/8" and the twin pot from the donor bike may have had 9/16", but he didn't know the specs of either bike.

Does anyone know the inside diameter of the master cylinder, I don't even know where to measure and I don't want to take it off right now.

There is also the option of going aftermarket if anyone knows of a good replacement, inexpensive salvage mc would also be a possibility but which bike should I be looking for?

The master cylinders are neither 5/8" nor 9/16". You need to tell your mechanic that you have a "metric" bike. …. and 9/16 is smaller than 5/8. Don't know what his theory is.
You would have the choice of a 14mm or 16mm.

Wyly, the first couple pages in this old thread might have some useful info.
Of course it then spirals into a brake fluid preference for 53rd time.
viewtopic.php?f=50&t=4738&hilit=master+cylinder+upgrade

thanks steve that gives me starting point

A Kawasaki Z -14 or C -14 master cylinder will give you 2 finger sloppy brakes with single pot calipers
EBay about $100
Best and easy mod

Thanks Artie I'll add that to my list of potential replacements to watch for.

EMS wrote:The master cylinders are neither 5/8" nor 9/16". You need to tell your mechanic that you have a "metric" bike. …. and 9/16 is smaller than 5/8. Don't know what his theory is.
You would have the choice of a 14mm or 16mm.

My cbx oem master cylinder is indeed 5/8" ...I'll match my "mechanic's" knowledge and his years on the motoGP and World Superbike circuit with anyone.

Indeed, it is 5/8". Don't know how/why on a Japanese bike. A 16mm reamer will not fit the bore. Ask me how I know.

Larry Zimmer wrote:Indeed, it is 5/8". Don't know how/why on a Japanese bike. A 16mm reamer will not fit the bore. Ask me how I know.

I wonder why Honda did that, I never paid any attention if the rear M/C was American Standard too or metric?

Bets hell out of me, Jeff. I 'discovered' it when I was rebuilding my front m/c. Measured it roughly, looked like 16. Bought a 16mm reamer. Guess what! ---- didn't fit!!! Got a piece of 5/8 drill rod for a gage -- fit perfectly. Whodda thunk. Have no idea what did that to Honda. Too much sake in the design department one day?!

Or they could have bought American industrial equipment in the post-war reconstruction years, and kept the size as a standard even after replacing the machinery for more modern stuff, as to ensure intercompatibility and continuity with previous productions.

Not sure about the front MC --- the service manual lists the "standards" as 15.870-15.913 mm, and 15.870 is 5/8". I'm just now rebuilding my rear master cylinder, and it is indeed 14 mm (standard there is 14.00-14.043 mm). A 14 mm flex hone (400 grit aluminum oxide) cleaned it up nicely with a finished bore of 14.02 mm.

The rear is indeed 14mm while the front is 5/8". Wonder who was the supplier? Really strange considering that there was quite a large market in Europe for these also. The part number changed for the 1979 at serial number 9212 to the same M/C as used on the non-US CB400N and then again for the 1980 to a Nissin part. The Prolinks shared the front master cylinder with the 1979-1981 CB750F ...go figure.