Master cylinder upgrade?

[wyly]

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?

[EMS]

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.

[steve murdoch icoa #5322]

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

[wyly]

thanks steve that gives me starting point

[Artie]

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

[wyly]

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

[wyly]

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.

[Larry Zimmer]

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.

[Jeff Bennetts]

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?

[Larry Zimmer]

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?!

[Jastreb_J21]

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.

[divan]

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.

[EMS]

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.