Bottom line up front: What am I looking for when selecting an aftermarket master cylinder with the intent being improved performance over OEM parts? How do you know one master cylinder is better than another? What specs are important?


I have been doing a lot of research and have solicited a lot of comments surrounding improvements to OEM single piston front caliper on an 06 FXSTS Springer. Several suggested upgrades throughout the equipment string. Most are easy enough to select. However, the master cylinder is one of those things I am not sure how to assess.

 

I am pretty certain that the aftermarket caliper manufacturers recommend fitment based on a certain bike type. They know the master cylinder bore and match their calipers to fit that particular bore. You specifically mention the master cylinder and there is a reason why things are sized the way they are. A MC too large in bore size will result in a hard brake lever/pedal...what is sometimes called a wooden brake. Likewise, one too small in bore size will result in a mushy feel. Neither of these is necessarily bad. You want a harder pedal on the rears and a softer lever on the front. You want to be able to "feel" and engage the fronts without lock-up. The rears...well when you hit the rears you're usually having to come to a sudden stop and the feel does not matter that much. Also, your leg is generally stronger than your arm. Manufacturers are now producing master cylinders for the front that are adjustable to suit the individual. Changing the size of the master cylinder bore is not always a good thing and can be down right dangerous if they are easily locked-up as soon as you squeeze the lever. There are master cylinder ratio charts available on line in which you can chart the ratio of one master cylinder size to another based on caliper volume. You do have to know the piston size of the caliper and the bore size of the master cylinder. Usually, Harley uses a 11/16 for the front brakes driving dual calipers and you can get by with a 5/8 on single caliper models. 3/4 bore can be had in some manufacturers lines. I did not mean to get this long, but in short, just changing the master cylinder size does not improve braking. It will certainly change it, but may make matters worse. A soft ratio is something like 18 and above and a hard pedal ratio is like 12 and lower.

My suggestion is to go ahead and purchase some of those high$ calipers and see how they work with your stock MC. If this is a ground-up build...I would go with the MC bore recommended by the manufacturer and you can find that on their web page. And...don't forget those stainless steel braided brake lines.