Maybe one of you can help me. My 1975 450 sl has a tick on the left side. I have replaced the cam, towers, cam oiler, rocker arms, lifters and valve stem caps. Still ticking. The engine also misses and I have replaced the cap, rotor, plugs and wires. My mechanic wants to pull the head because he thinks the valve guides are loose. The engine holds 160 pounds of pressure +/- 10. Before he takes my engine apart, what else can I check? Thanks!