Setting the Valve-to-Rocker Clearance on a 1968 Triumph Bonneville T120R
Correct valve-to-rocker clearance is important on the 650cc Triumph Bonneville T120R. The adjustment should always be made with the engine completely cold. For the 1968 T120R, the specified cold clearance is 0.002 inch (0.05 mm) for the inlet valves and 0.004 inch (0.10 mm) for the exhaust valves.
Begin by removing both spark plugs. This eliminates compression and makes the engine much easier to turn. Remove the rocker-box inspection caps to gain access to the square-headed rocker adjusters and their locknuts. The factory workshop procedure recommends turning the engine so that the valve opposite the one being adjusted is fully open; this places the rocker being checked on the base circle of its cam.
Insert the appropriate feeler gauge between the rocker adjuster and the valve stem. For an inlet valve, use the 0.002-inch gauge; for an exhaust valve, use the 0.004-inch gauge. The gauge should pass through with a slight, consistent drag.
If adjustment is necessary, loosen the adjuster locknut while holding the square adjuster. Turn the adjuster until the correct clearance is obtained, then hold it in position while tightening the locknut. Always recheck the clearance after tightening the locknut, because the setting can change slightly.
Repeat the procedure for all four valves. Rotate the engine carefully and check each valve at the correct position rather than relying simply on piston position. Once all four clearances have been confirmed, replace the inspection caps and spark plugs.
The factory manual recommends checking the rocker clearances periodically, with 3,000 miles (4,800 km) given as a service interval.