Rim brake pad interchangeability?
I've made some observations checking out different bikes here and there and at today's workshop for Christmas Anonymous --

We will assume, for the sake of this discussion that brakes fall into three categories, calipers, cantilevers, and V-brakes.  (Yes, there are others, but not common enough to worry about unless you're really into odd bikes.)

Calipers appear to (almost) always use simple pads with threaded studs.  I'm vaguely recalling from the past that there are some variations on this scheme, but I haven't observed them of late.

I have observed at least four variations for cantis:


(Updated) A pad with a female thread, designed to accept an Allen-head bolt.  This pad appears to be very much similar to the pad for V-brakes.
A pad with an unthreaded stud with striations running parallel to the stud's length.
A pad with an unthreaded stud with "ribs" (shallow grooves) about every 2mm along the stud's length.
A pad with a smooth unthreaded stud.


(Note that I've been shown/sold pads of both the #3 and #4 variety from local shops, and both are labeled simply as "canti" pads.  It's not clear how one differentiates.)

From what little I can tell, there is a slight variation in diameter between the three varieties of unthreaded studs.  The stud with parallel striations (#2) has the smallest diameter, while the smooth stud (#4) has the largest diameter.  The #3 is somewhere in-between.  This variation is probably not enough to make a #2 unstable in the mount for a #4, but a #4 stud will not fit in a #2 mount, and a #3 stud is borderline fitting into a #2.

(Updated) V-brakes appear to always use a brake pad with a female thread, designed to receive an Allen bolt.  On the bolt are typically washers and bushings that allow the pad to swivel, after a fashion, for adjustment.

(Everyone, of course, is encouraged to reply with your specific observations on this topic.)