My friend just rebuilt his s4 na, and he lost compretion on the rear rotor after only running for about 5min. so he took it back apart and found that the sides of the back rotor around the tips of the rotor were scratched up and burned.I will try and get some pictures of the rotors this week so you guys can see more clearly whats happening,but all the corner seals and the ends of each side seal are ceased in there grooves,and the side housings for that rotor are burend as well.he didnt properly clearens the side seals he just sanded them down untile they fit,and he didnt check the running clearenses for the rotors.I think it might be one of those to factors or that these are all used seals and differant parts from differant engines that wornt checked at all.Im prity shure the rotors used wornt even the same weight

 

with everything you just stated its a recipe for disaster, but if the rotors happen to be the same weight by a miracle and he somehow rebuilt it right and everything was good he could have over torqued the engine bolts and that made the rotor completely to compress the rotor and seals to the point of failure thats why they are ceased up.

 

I dont know what he tourqed the engine down to it could be posibule.I was thinking with his side seals being posibly to tight they may have expanded when they got hot, or if the rotor was to tight for that houseing it may have flexed around and the tips touched the side housings.

 

sounds like a rotor housing that was decked. if the rotor housing thickness is well below specification then the rotor is compressed and tolerances below minimum. as the rotor spins it needs some clearance for the side seals to take up the slack, if there is no slack then the rotor itself runs on the surface.

only other thing that causes this sort of failure is lack of lubrication.

i have an engine in the shop right now that has a similar issue but i have not checked the running tolerances of the engine yet. but i believe this engine was also used at high RPMs for an extended period of time which can cause the corners of the rotors to contact the irons if the rotors are not race clearanced.