Tappet Clearance is nothing but a clearance (gap) between rocker arms and the point where they rest on valves. Both surfaces are flat.
This small clearance is essential between tappet and valve stem when the valve is in closed position.
During operation, with the rise in temperature, the valve stem expands and therefore, the clearance must be sufficient when engine is cold. Clearance is essential to allow thermal expansion of valve spindle during operation.
General clearances are between 0.3 mm to 1.5 mm depending on manufacturer and valve.
In general, exhaust valve clearance is more as compared with inlet valve clearance but not necessarily. On my ship, both valve has same clearances.
Excessive clearance is not desirable as this may lead to hammering of the tappet on the stem and make the engine noisy and may result in damage of parts.
Clearance & setting must be carried out on the cold engine.
Which unit is considered for tappet clearance that unit must be at ignition TDC. TDC for each cylinder is marked on flywheel. Just turn the flywheel and make sure the required unit is at TDC.
For 6 or 8 cylinder engine, at one time two unit will be at TDC. One unit will be in power stroke and other will be in compression stroke. Tappet clearance is always taken in compression stroke, As at compression stroke inlet and exhaust valves are closed. When valves are closed, both rollers of the valve drives must sit on the base circle of the cam.
The crankshaft is to be turned through flywheel in such a way that the desired cylinder unit piston comes at ignition TDC (compression stroke)
To make sure that one unit is at compression stroke, shake the push rods of both units by hands to check which is free.
Unit with free push rods is at compression stroke, avoid power stroke unit which has tight push rods.
Now, good filler gauges of different thickness has to be pushed gently between clearance of valve and rocker arm.
Check standard clearance of manufacturer and compare.
Now, if clearance is not matching i.e. more or less then we have to do adjustment.
While making adjustment, feeler gauge should be in between clearance and to be removed after tightening or slackening.
Why clearance increases or decreases?
During operation of the engine, there is some amount of wear of the valve lid & seat and this may cause the valve stem to move up i.e. the tappet clearance to reduce.
Clearance can increase due to wear of the cam, roller or tappet surface or mechanism.
Consequences of incorrect tappet clearance
“Tappet clearance is more : valve will open late and close early”
If air inlet valve opens late, it will not be sufficiently open when the piston commences its suction stroke causing some wire drawing and reduction in volumetric efficiency.
If air inlet valve closes early, there may be some wire drawing towards the end of suction stroke and moreover full advantage can’t be taken of the inertia effect of the moving mass of air to fill the cylinder with maximum amount of air before the start of compression.
“Tappet clearance is less : valve will open early and closes late”
If air inlet valve opens early, the piston may pump some of the exhaust gases into the inlet manifold which may return to the cylinder during the suction stroke.
If air inlet valve closes too late, some air may be pumped back to the inlet manifold.
In all situations, amount of fresh air available will diminish that will affect combustion badly.