HELP!!!! I've searched the forum with no luck. How do you adjust the valve clearance on a 1975 450SL? From what I've researched, there is a threaded bushing that threads into the head, and a valve adjusting screw that threads into the bushing. From what I've read, you simply turn the adjusting screw to adjust the clearance, and there is a built-in friction in the bushing that keeps the screw from loosening. No lock nut to loosen.

I've tried to turn the adjusting screw on several valves with no luck. They are extremely tight. What am I doing wrong?