To find top dead center for any cylinder, rotate the engine until the fuel intake valve is almost all the way down (open); at this point the piston is at the bottom of the inlet stroke. Put a chalk mark on the flywheel. Now rotate the flywheel another half revolution. At this point the piston in that cylinder will be at Top Dead Center.
Check the clearance on the valves for this cylinder and decide if adjustment is required. It is not always necessary to adjust all the valves, so always check the clearance of each valve before making any adjustment. If the engine manufacturer says that the valve clearance is to be 0.008 inch, then you should be able slip a 0.008 feeler gauge through the tappet clearance. But you should not be able to slip the 0.009 gauge through.
MAKING THE ADJUSTMENT
If adjustment is required, loosen the lock nut of the valve to be adjusted and turn the adjustment screw until achieving the correct clearance, as measured with the feeler gauge.
American-built engines have two types of tappet adjusting screws. One is the self-locking. the other, which is more common, has a locking nut to hold the adjusting screw in place. This locking nut must be loosened and the adjusting screw or nut held in place while it is re-tightened after the adjustment is completed. On most American engines the adjusting screw or bolt penetrates a threaded hole in the rocker arm.
CAPT. ALAN R. HUGENOT
Foreign-built engines have a number of different arrangements. The Swedish-built Volvo marine diesel does not have a screw head that penetrates the rocker arm on the side opposite the spring. Instead, it has a bolt head that resides on the lifter below the rocker arm, opposite the spring. This bolt threads up from the lifter, and has an adjusting lock nut on the top of the lifter. The lifter also has two flats to hold a wrench. (See photo)
After the valve clearance has been adjusted and re-checked, tighten the lock nut.
Since you are at Top Dead Center, you can now adjust the other valve on this cylinder as well because both valves are closed. Simply repeat the steps for the other valve. It doesn't matter whether you do the intake valve or the exhaust valve first.
Perform the above four steps for all cylinders, by rotating the engine until the next cylinder in the firing order comes to Top Dead Center. When you are finished, reinstall the rocker arm cover using the undamaged gasket (or a new gasket), using high-grade gasket cement. Tighten all the cover bolts as recommended by manufacturer.
Capt. Alan Hugenot is a naval architect and marine surveyor based in San Francisco,whose writing has appeared regularly in Sea Magazine, Latitude 38, The Log newspaper, 48 Degrees North, Go Boating and many other boating publications on the Pacific coast. He serves as National Chairman of the Motor Yacht Technical Committee for the Society of Naval Architects and Marine Engineers.
