The exhaust guide in the M20 is longer than the inlet and the top 5/16" or so is counter bored to about .040" oversize (that's an approximate figure subject to accurate measurement) to prevent friction caused by contact between carbon build up on the valve stem and the top of the guide...
Interestingly, heat transfer from the valve head (which is red when running) through the stem to the cam followers must have caused problems with wear at the foot of the follower as the follower foot is hard chrome plated (the inlet is not)..Presumably the heat treatment was affected...OHV engines such as the B31/33, which have pushrods, do not have this treatment applied...Ian
