What Size Master Cylinder Do I Need?

We’ve heard iterations of this question many times. The answer is a rather unfortunate one, though: we don’t know.

It’s not a cop-out answer, but a reality. The “feel” of a brake system is highly subjective. If your master cylinder is “too big,” so to speak, you’ll get full stopping in a very short throw of the lever, but you’ll need Popeye-like forearms to make that happen on a front brake. (A rear will just feel like a light switch, either on or off.) 

Too small a cylinder yields the opposite problem: you have a lovely, progressive application that feels like power brakes in a car, but you might run out of lever travel on a front brake. At the rear, this often manifests itself as a boot coming to a stop because our ankles don’t have very much flexion or extension. (Try it while you’re sitting there reading this; normal range is only about 45° down and only about 20° coming up.)

If we go back in history, it’s clear that engineers were also learning about this relationship between bore sizes; it’s why some brakes on some bikes feel kind of horrible even when everything is mechanically (and hydraulically) in good condition. 

Happily, there’s a little shortcut you can take to zero in on the right stuff if you are combining things off different bikes. It’s pretty simple math. You need to calculate the surface area of the master’s piston and the surface area of the caliper piston. Multiply the number of caliper pistons by the surface area, then set it up as a ratio. (Remember geometry in high school? Πr² is the formula you need to calculate piston area. It also helps to know there are 25.4 mm in one inch.)

That super-two-finger braking you get on a sport bike? That’s something like 27:1. The wooden early 70’s lever feeling? That’s maybe half that at 13.5:1 or so. 

Now that you have an idea of the range, let’s do some quick math on a real-world example.

Let’s pretend we wanted to use a pair of dual-piston calipers from a middleweight sport bike on the front of a Harley equipped with a stock 5/8” master cylinder. Shortcutting through the math, we’d get 2,290.24mm of surface area at the brake calipers, and up at the master we have 197.93mm of surface area. That yields 11.57:1, which would probably feel pretty firm—probably too firm for most people. A smaller master cylinder bore would give a much better ratio.

Now this isn’t a hard science, since the diameter of the rotors (bigger gives the brake more leverage) works against the diameter of the wheels (smaller gives the brake more leverage). Both affect the feel of the system. Don’t forget that the mix of the puck compound used in your pads as well as the mechanical advantage your brake lever or pedal offers plays into the equation as well, and finally, brake feel is also somewhat a matter of personal preference. But by working out the math behind some combinations you’ve used and liked in the past, you can usually calculate a ratio that’s going to be in the ballpark of being acceptable. It’s way cheaper—and way less work—than just trying different combinations of bore sizes. 

Working backwards, you may find it helpful to bounce between SAE and metric measurements; you’ll find some combinations are quite popular and others much less so. In our previous example, a common late-model Harley Davidson master cylinder uses a piston with a 1/2” bore, which is acceptably close to 12mm, and would yield an 18.07:1 ratio—much better. And, of course, it would be much easier to find and buy rather than looking for some esoteric combination that may not be widely produced.

So while we might not be able to tell you exactly what to order, you’re now well-armed to tell us what you need!