Tappet clearance is a space between the top of the valve stem and the rocker arm. Its purpose is to allow for some mechanical expansion and lengthening of the valve stem and push rods as the engine warms up. This clearance is also called valve lash.When checking tappet clearance on marine engines, we have to ascertain that the piston is at TDC. this can be ascertained by following methods 1.Flywheel Method: -
The flywheel is the simplest method to know which unit is at TDC. If the flywheel shows two units, simply open the bonnet covers and checks 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.
2. Dial Gauge Method: -
In this method the fuel injector is taken out and from the opening a dial gauge is put inside. Then the turning gear is engaged and the engine turned over. The pointer of the dial gauge will move in one direction and then stop and start in opposite direction. The moment the pointer of the dial gauge stops
and changes its direction of movement is the TDC of the unit. This method is not normally used in day-to-day practice, but may be used in the calibration of the flywheel if it is not calibrated, or after some repairs.
3.Camshaft Method: -
The camshaft window of the engine can be opened up and the camshaft inspected. cam has a base circle and dwell period, when at base circle, cam valves are open or piston at TDC.It must be remembered that as a four-stroke engine has two rotations of the crankshaft there is one injection TDC where the injection and the combustion take place. The second time the piston is at TDC is when the exhausting of the flue gases takes place. It is very important to identify the combustion TDC, as tappets have to be adjusted at that point. camshaft rotates once for every twice rotation of crankshaft
4.Crankcase Method: -
In this method the crankcase doors are opened up and the piston is visually checked whether is going up or down. This is the surest method, but a bit cumbersome. It should be used when you have a strong doubt about the other methods.
(exhaust valve tappet value-.4mm suction valve- .3mm)
The flywheel is the simplest method to know which unit is at TDC. If the flywheel shows two units, simply open the bonnet covers and checks 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.
2. Dial Gauge Method: -
In this method the fuel injector is taken out and from the opening a dial gauge is put inside. Then the turning gear is engaged and the engine turned over. The pointer of the dial gauge will move in one direction and then stop and start in opposite direction. The moment the pointer of the dial gauge stops
and changes its direction of movement is the TDC of the unit. This method is not normally used in day-to-day practice, but may be used in the calibration of the flywheel if it is not calibrated, or after some repairs.
3.Camshaft Method: -
The camshaft window of the engine can be opened up and the camshaft inspected. cam has a base circle and dwell period, when at base circle, cam valves are open or piston at TDC.It must be remembered that as a four-stroke engine has two rotations of the crankshaft there is one injection TDC where the injection and the combustion take place. The second time the piston is at TDC is when the exhausting of the flue gases takes place. It is very important to identify the combustion TDC, as tappets have to be adjusted at that point. camshaft rotates once for every twice rotation of crankshaft
4.Crankcase Method: -
In this method the crankcase doors are opened up and the piston is visually checked whether is going up or down. This is the surest method, but a bit cumbersome. It should be used when you have a strong doubt about the other methods.
(exhaust valve tappet value-.4mm suction valve- .3mm)
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