When an engine loses compression from bad seals, have the seals actually worn down to where they don't protrude from the rotor? If not, why do they lose sealing ability? I would think that once they are broken in, the frictional wear would keep the seal surface and the housing mated with the same shape and sealing good until the seal itself was worn down to the rotor surface. A similar question - I pulled a clutch that was slipping badly and the plate still had material on it - Was the slipping from something else or can a clutch "wear out" and still have material on it? If so, how does this work?

Logically, first place to look when it comes to losing sealing ability as seals wear out, is what forces the seal to have the sealing ability in the first place



The spring will lose tension as the apex seal begins to lose thousandths of an inch, the spring becomes not so tense





But that's my educated guess, I've torn them apart a few times, I haven't built 100s.

yeah, i've heard that too, it's not just seal wear, but those springs which have remained tense for years they eventually just wear out and don't work as good

also the housing surface deteriorates, they start life flat and smooth, the more imperfections in the chrome surface the worse its gonna seal.

Alot of compression loss can be attributed to carbon sticking of the seals. Any carbon that forms in between the seals and rotors will increase the clearance tolerances causing the seals not to move as freely. Carbon is your worst enemy when it comes to having good compression in a rotary(especially a high mileage one).

carbon sticking

slight deterioration

springs losing tension

improper wearing (corners of side seals)

improper wearing of the apex seal (bad exhaust porting)

breakage

scratching and/or chrome flaking of the rotor housings

grooves widened to trapezoid shape--old rotors

worn bearings allowing the rotor to move just a little too much

etc, etc, etc