Is it possible to use a 23mm master cylinder on a stock 914/6 without having to make any modifications. Will it just bold in? Is this overkill? I need to replace the original 19mm and was thinking that it might make for a nice upgrade?

do some searching here. unless you have MONGO 4 piston brakes everywhere or just want a NO MOVEMENT pedal, it is overkill. a 19mm with stock or late model 2 piston calipers is sufficient

requires a new pushrod IIRC

Thanks Aaron!

I have a 23mm master cylinder on my car, and even with 4 pistion 944 turbo calipers all the way around the pedal is high and hard.

I may go back to a 19mm to make it have more pedal travel (better feel).

If you are running stock brakes, the 19 will be fine.