Hey guys - happy new year!

So, I'm trying to compare two master cylinders to see if the standard Marina one will serve the new Capri calipers and drums.

The Marins MC is:
19mm bore
37mm stroke

The Capri one is:
19-20mm bore (close enough?)
35mm stroke

Now, I've taken the bore size without taking the piston out, just measuring the top part of the piston up to the edge of the cylinder - is that accurate enough?

Also, how close do they need to be, ie is my 19-20mm measurement good enough or should I remove the piston to get a good measurement.

Any help appreciated
Rob