I have an 1975 450 sl with a mysterious engine tick. After replacing the left cam, rocker arms, cam towers, lifters and valve stem covers/seals my mechanic has determined that it must be the valve guides. I burn a little oil when I first start the car and the engine is missing (also replaced the cap, rotor, plugs, and wires) and ticking. The engine is at 160 compression +/- 10 which is good. He claims that the burning oil is from the rings and that if he does a valve job the increased compression will blow the rings. I think the burning oil is from the loose valve guides and 160 is about as good as it gets for compression. Anybody have some advice? Thanks!