With the piston and crank at T.D.C. both the inlet and exhaust valves will be closed as the cam follower is on the back of
the cam, you are in a position to adjust the valve to rocker clearance.
Check the gap with feeler, if to tight or loose,loosen locknut, adjust the gap by turning the screw to reduce or enlarge
the gap.Ideal setting is where the feeler can be moved smoothly back and forth through the gap. When done hold the adjusting
screw whilst tightening the locknut. Recheck the gap to ensure the gap is correct, this may take a few attempts.....