How Critical is getting TDC exactly right?
• The fact is, you don’t need to be EXACTLY on TDC for both cams to check clearances. If you ARE, then you WILL be checking it correctly. The cam is egg shaped. The “round end” of the cam, like an egg, has the same profile for about 45 degrees of cam rotation. You can check timing on EITHER cam through 90 degrees or so of cam rotation. But the timing for closed valves at TDC, where the clearances must be checked if both in and out are to be checked at once, is considerably less. Nevertheless 15 degrees or so of cam rotation (I seem to recall) for an off-tooth, should still allow an inobservant mechanic to check clearances. Of course if any of the shims HAD been changed at the last service, I would suggest that you had paid good money to have a bad mechanic screw up the timing on your bike. Yet another reason to do valve shim changes yourself.
