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.
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.
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