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.
If insufficient valve lash is set when the engine is cold the valves will not properly close when the engine warms up. If too much lash is provided (additional clearance) then even after the engine warms up there will be some clearance, which will result in lost motion. Lost motion mean that as the cam tries to open the valve the push rod and rocker arm moves to first take up the clearance before touching the valve to open the valve. The result is late opening of the valve.
When checking tappet clearance on marine engines, we have to ascertain that the piston is at TDC. Though markings are provided on the flywheel, the marine engineer must know the other methods for this like inspection of the camshaft and the fuel pump window.
During the maintenance of a four stroke marine diesel engine there are times when we must know whether the particular unit’s piston is at the top dead center of not. For example when checking the tappet clearances of the engine it is important to know which unit is at TDC.
Referring to the flywheel would indicate two units, but only one can be at injection TDC. So which one is it?
If insufficient valve lash is set when the engine is cold the valves will not properly close when the engine warms up. If too much lash is provided (additional clearance) then even after the engine warms up there will be some clearance, which will result in lost motion. Lost motion mean that as the cam tries to open the valve the push rod and rocker arm moves to first take up the clearance before touching the valve to open the valve. The result is late opening of the valve.
When checking tappet clearance on marine engines, we have to ascertain that the piston is at TDC. Though markings are provided on the flywheel, the marine engineer must know the other methods for this like inspection of the camshaft and the fuel pump window.
During the maintenance of a four stroke marine diesel engine there are times when we must know whether the particular unit’s piston is at the top dead center of not. For example when checking the tappet clearances of the engine it is important to know which unit is at TDC.
Referring to the flywheel would indicate two units, but only one can be at injection TDC. So which one is it?
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. There is a word of caution however: this method is only useful in a working generator, which you have just stopped to check the tappet clearances. In case you have removed the rocker arms for any reason the spring height and the push rod freeness check would lead you nowhere and misguide you.
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
Camshaft Method: -
The camshaft window of the engine can be opened up and the camshaft inspected. The cam of the engine has a base circle, and acceleration and dwell periods. If the roller of the follower is at the base circle, then the particular valve is closed by spring action. When both the exhaust valve and the inlet valve follower are on the base circle, then the unit is also 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.
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.
Valve Spring Method: -
This is not an independent method but is used in conjunction with the flywheel method. In this method if the flywheel is indicating two units, you can check the springs of both the units. The unit in which the springs are loose is the one at TDC. The caution is that this method is useful for an engine in use. If you have removed the rocker arms during the overhaul and thereafter you want to use this method then it can cause errors.
Push Rod Method: -
This method is like the spring method and you check that the push rods are free to turn. The unit at TDC will have loose springs. The care that must be taken is that it should be used along with the flywheel method and should be used in a working engine. By a working engine, I mean the engine that was running and has been stopped for tappets adjustment.
Loosen the lock nut of the rocker arm.
Now adjust the tappet clearance between the rocker arm & valve stem by tightening or loosing the nut below the lock nut.
If tappet clearance is less:
I. Valve will open early & close late
ii. Air induced through inlet valve may leak out. So less air for combustion.
iii. Power will be reduced.
iv. Fuel consumption will increase, engine may become unbalanced, exhaust temp. will be very high.
v. In worst condition, valve may remain open, resulting in loss of compression pressure, burning of exhaust valve, T/C fouling will increase.
If tappet clearance is more:
I. Valve will open late & close early.
ii. Lesser heat energy to T/C, so reduction in scavenge air & hence power.
iii. No proper removal of gases.
iv. Hammering of valve stem-may cause damage to valve stem.
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
Camshaft Method: -
The camshaft window of the engine can be opened up and the camshaft inspected. The cam of the engine has a base circle, and acceleration and dwell periods. If the roller of the follower is at the base circle, then the particular valve is closed by spring action. When both the exhaust valve and the inlet valve follower are on the base circle, then the unit is also 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.
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.
Valve Spring Method: -
This is not an independent method but is used in conjunction with the flywheel method. In this method if the flywheel is indicating two units, you can check the springs of both the units. The unit in which the springs are loose is the one at TDC. The caution is that this method is useful for an engine in use. If you have removed the rocker arms during the overhaul and thereafter you want to use this method then it can cause errors.
Push Rod Method: -
This method is like the spring method and you check that the push rods are free to turn. The unit at TDC will have loose springs. The care that must be taken is that it should be used along with the flywheel method and should be used in a working engine. By a working engine, I mean the engine that was running and has been stopped for tappets adjustment.
Loosen the lock nut of the rocker arm.
Now adjust the tappet clearance between the rocker arm & valve stem by tightening or loosing the nut below the lock nut.
If tappet clearance is less:
I. Valve will open early & close late
ii. Air induced through inlet valve may leak out. So less air for combustion.
iii. Power will be reduced.
iv. Fuel consumption will increase, engine may become unbalanced, exhaust temp. will be very high.
v. In worst condition, valve may remain open, resulting in loss of compression pressure, burning of exhaust valve, T/C fouling will increase.
If tappet clearance is more:
I. Valve will open late & close early.
ii. Lesser heat energy to T/C, so reduction in scavenge air & hence power.
iii. No proper removal of gases.
iv. Hammering of valve stem-may cause damage to valve stem.
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