So I’ve been away from riding/racing for about 6-7years now and my new to me bike is in need of a master cylinder. So to keep it simple can someone explain the difference between the 19mm masters vs the 17mm. Bike in question is 2015 R1 with stock calipers.
^^ LOL the short answer is, if your calipers are the same, and your pivot distance and lever are the same - the 19mm bore master cylinder will move more fluid for the same movement of the lever. More fluid movement means more piston movement. So you will have a higher effort at the level with a shorter pull on the 19mm and a lighter effort on the lever with a longer pull on the 17mm to move the brake pistons the same distance.
I would add, bigger brakes will need more fluid moved therefore a larger master cylinder. A little single brake Kawi 400? Maybe a 15-17mm master cylinder. A big R1? I would probably use a 19mm. My Kawi 650 uses a Brembo 19mm and my Aprilia RS660 a 17mm and I do like the feel of the 17mm more. But I assume the brakes on the R1 are larger than the Aprilia.
I wasn’t willing to commit the time to go down to number of pistons and piston diameter and ratios and do all the math and such. As with anything on the Internet, accept or reject inputs as you see fit! I was happy with the 17mm M/C feel on the Aprilia even though I had been running 19mm M/C on my prior dual brake 4-piston brakes for LWT twins. But I did used to run GSXR 1000 brakes on my SV so those “bigger” brakes matched up well with the 19mm.
You don't need to do the math to define a term. In the case of hydraulics and your post, "bigger" means more caliper piston area, and usually only of one side of each caliper. So for a 4 piston caliper, you'd add up the area of only the 2 left-side pistons for example. "Bigger" does not mean larger rotors, larger brake lines, a bigger bike, etc etc. It doesn't even necessarily mean more pistons, as you'd need to measure and may ignore half the pistons.
