When checking tappet clearance on marine engines, we have to ascertain that the piston is at TDC. Though markings are provided on the flywheel, we must know the other methods for this like inspection of the camshaft and the fuel pump window.
Referring to the flywheel would indicate two units, but only one can be at injection TDC. So which one is it?
The flywheel is the simplest method to know which unit is at TDC. If the fly wheel shows two units, simply open the bonnet covers and check visually. The unit at TDC will have both the inlet and the exhaust valve closed and hence relaxed springs. The other unit would have both the arms of the rocker arm at different levels. In addition the push rods of the unit at TDC would be loose and can be turned by hand because of the release of the clearances.
Note: This method is only useful in a working generator which you have just stopped to check the tappet clearances. In case we have removed the rocker arms for any reason or after overhaul, this method is not useful.
Fuel Pump Method
The most accurate method to know the position of the piston without opening up the piston is the fuel pump window. The fuel pumps have a window and as the plunger goes up and down, so does the mark on the bottom spring holder. On the body of the fuel pump there are cut marks which show the start of injection. In a diesel engine the start of injection is the injection TDC where both the inlet and the exhaust valves are closed. In this injection TDC we can check the tappet clearance. It must be noted however that the injection TDC is not the absolute TDC as the piston is still some way down depending on the design of the engine. An injection TDC may be around 5 degrees before TDC.