12.5mm vs 14mm Piston Bore (Master Cylinder)

Looking to update and upgrade my master cylinder on my bike. I want something new that has a built in switch for the brakes to rid the one on the tree of my bike.

My bike is a 1974 Kawa KZ400 with a 12.5mm piston bore and most of what I see out there is 14mm.

How much of a difference will this make?

Less lever travel, more grip force needed to get the same level of braking...

The amount of force of at the caliper is the amount of pressure you put at the lever, multiplied by the ratio of the areas of the caliper piston(s) and the MC piston.

So for arguments sake, lets say the stock MC piston is 1 square inch in area, and the caliper 6 square inches. The lever itself has about a 6 to 1 lever ratio.

So, you squeeze 10 lbs on the lever, the lever multiplies that to 60 lbs.
The MC to caliper multiply by 60 again, so you have 360 lbs of force on the brake pads.

The movement of the lever is reduced the same way the force is increased, so the end of your lever moves 36 times further than the piston moves.

If you use a 1/2 square inch area MC piston, then the force at the caliper is 720 pounds.

If you use a 2 square inch MC piston, then the force at the caliper is 180 lbs.

So, the area of the original piston is Pi/4 x D^2 = 122.7 mm^2
The new MC is 154 mm^2

So it will take approx 25% more lever pressure to get the same stopping force.

The lever will move 75% of the distance it did before.

So if the end of the lever moved an inch before, now it will only move 3/4 of an inch

If it took 20 lbs to lock the brakes, now it will take 25lbs.

Basically, it will be harder to stop. Less effective braking because you need more hand pressure.

Brakes that only needed 2 fingers to stop would require more fingers...LOL

I would try to stay the same....I think 11mm is available, and that would give you more braking power.

Mike's XS has 11, 12 and 13 mm master cylinders.

FWIW, I'm going to try a M/C that is advertised to be 10 mm bore on my CB360.

So with a 13mm or smaller the lever would require more pull to engage the brakes but would brake harder.

Looks like MikeXS has a 11mm which from what mydlyf was saying is that it would require more travel to apply pressure but would be applied harder.

Learned something new!! Always thought bigger piston would be an upgrade. Good thing I haven't placed my order yet.

There are other factors to better braking. Surface area of the brake pads, friction coefficient of the pad material and friction coefficient of the rotor metal. Stainless is not as good as cast iron for stopping. So a bigger or smaller mc piston is only a small part of the braking system.


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Here is chart on master cylinder ratios: http://vintagebrake.com/mastercylinder.htm I used it to pick a master cylinder for my '75 XS650 with dual discs, and it worked great.

Well...the M/C I received is definitely 14 mm, so stock size it is. Need to see how it works and might downsize later on.