19x18: This specification indicates two values. The first value is the diameter (ie. bore) of the cylinder in millimeters - it is usually 16mm or 19mm. In this case, the diameter is 19mm. The second value is the inter-axis (ie. distance) between the lever's pivot point and the plunger that pushes into the cylinder - it is usually 16mm, 18mm or 20mm. In this case, it's 18mm.
Now that we know what the numbers are, let's figure out what they mean in terms of braking performance. When you are selecting a master, you need to understand that these values trade-off braking sensitivity and braking power.
For the cylinder diameter, as that value increases, you increase your braking power. As you increase the diameter, you increase your cylinder size and increase the volume of brake fluid that you have to compress. This creates a dampening effect that allows you to better modulate the amount of brake pressure. As a general rule of thumb, you would use a 16xXX for a single caliper set-up anda 19xXX for a dual caliper set-up. Of course, there are always exceptions - for example, the stock master cylinder for Yamaha R1's and R6's (which are made by Brembo) use a 16xXX set-up, despite the fact that they have dual front calipers.
For the inter-axis value, as that value increases (ie. the distance gets longer), you are decreasing your sensitivity and increasing your brake power. I don't want to get into the technical aspect or into the physics of it...that's not the goal of this article. If you feel like you need to know more, I would recommend you search Google or How Stuff Works. In a general comparison between a 19x18 and 19x20 configuration (the most common configurations for sportbikes), a 19x18 has more feel but has a little more lever travel than the 19x20. A 19x20 configuration has more braking power and requires less distance to completely pull in the lever.
Ultimately, the optimal configuration is up to you. Brembo recommends the 19x18 configuration for racers and the 19x20 configuration for street riders. In terms of real world examples, the billet master cylinder using in MotoGP is a 19x18 while the master cylinder includes with the Brembo High Performance street kits is a 19x20.
Last edited by Stealth : 29-Jun-2009 at 07:00 AM.