20 degree chamfer. What are they referring to? Any illustrations greatly appreciated.

 

Make a ~4" long right triangle with one 20 degree and one 70 degree angle out of a 3x5 card. Lay its hypotenuse across the 'cuts' in the piston face, and rotate the piston until the base of the triangle is perpendicular to the long axis of the caliper.

Right front:



Right rear:

 

Thanks, very helpful. One 928 repair manual I have says the angle is reversed on the rear caliper's piston? Is this true, does it make sense?