The clearance in between the valve stem and rocker arm is termed as tappet clearance. They facilitates the operation of rocker arm in four stroke engine; by providing sufficient space for thermal expansion. Tappet clearance is taken when the engine cools down and both exhaust and inlet valve under closed position i.e compression stroke.
There is no real mechanical linkage between the valves and the rocker arm in four stroke engine. When you open the cover of cylinder head for inspection, you will find a gap / clearance in between. There is no physical connection; valves are operated by the movement of push rod operating ( pushing ) rocker arm in one or another direction.
When there is a change i.e “increase or decrease” in this clearance due to thermal or mechanical expansion; valve timing are adversely affected so are the power cycle. An increase in clearance will lead to late operation and early closure of exhaust and inlet valves. A valve that will open late and close early will have adverse effects on the engine; such as loss of power, improper combustion, hammering, high exhaust temperature and engine failure.
On other hand a decrease in tappet clearance will lead to early operation and late closure of valves. A valve that will open early and close late will have negative impact on the performance of the engine; such as power loss, leakage of scavenge air, high fuel consumption, heavy shoot formation, loss of compression pressure and even burning or total valve failure.
So tappet clearance are regularly measured and adjusted based on manufacturer data to ensure positive operation of inlet and exhaust valves.
Taking tappet clearance for an engine unit is easy but skillful task. Ensure that the unit is on the TDC, turning the turning gear clockwise up-to the appropriate markings. Re-insure about the position by checking the fuel cam as well as freeness of push rod. The engine unit is kept at TDC or compression stroke to ensure availability of both exhaust and inlet valves for measurement of clearance.
Checking freeness of push rod ensures that the unit is on compression stroke and not on power stroke; as in four stroke engine two units are at TDC in any given time. Now, once you are ready to measure the tappet clearance; go through your manufacturer manual to ensure the correct clearance settings.
Try to insert filler gauge of different size in between the gap of valve steam and rocker arm. Select different set of filler gauge until you find the right one; which just pass through the tappet clearance without much resistance. A gauge that can’t pass is just too big but no resistance means the size of gauge is small so try another one. Once you find the right gauge; look for the dimension marked on it. Now compare it with the manufacturer recommendations; Adjust if necessary.
Now after correct measurement of tappet clearance; you know whether to increase or decrease to set it at manufacturer recommendation. All you need is to adjust the clearance; but how? Tappet clearance is adjusted after opening the lock nut above the rocker arm. This lock nut restricts movement of tappet to ensure proper clearance setting. Open the nut using spanner of appropriate size ( Normally Ring Spanner are Used).
Once the locknut is disengaged; tappet is free to move up and down. Adjust the tappet clearance by adjusting the position of tappet. To ensure correct clearance setting; it is a common practice to place the filler gauge of desired dimension in the clearance area during adjustments. Make sure you can slide the filler gauge smoothly once it is set; Too easy to pass is an clear indication of high clearance. This is sometimes hard to judge making it a job of skill and experience.
A small turn will ensure movement up to one hundredth of millimeter. Place the lock nut in place once done; re-tighten it without affecting the new clearance setting by accidental movement of tappet. Recheck the clearance if necessary and made a record of maintenance for future purposes. Make sure all is right otherwise any mis-adjustment can lead to severe conditions such as push rod bending or valve failure.
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