Here,we will discuss about the importance of Tappet Clearance of the Inlet and Exhaust valve in four- stroke Diesel engine.

What is Tappet Clearance ?

Tappet clearance is a space between the top of the stem of the valve and the rocker arm when the valves are closed position & the engine is at the cooled condition at the compression stroke.

Some Other Definitions are :

This is the space /clearance available between the tip of the rocker arm and the valve stem when the valves are closed when the engine is in cooled conditions , i.e. in compression stroke.

It is nothing but a gap between the rocker arms and the point where they rest on the stem of valves. These Both surfaces are flat.

Note :- It is also known as valve lash (in some book ).

When Tappet Clearance is Taken ?

It should be taken

1. After certain running hours ( As given in manual ).

2. When sound coming from tip of rocker arm.

Conditions for taking it…

It is taken when the inlet and outlet valves are in closed position & engine at the cooled condition at the compression stroke.

What is the purpose of Tappet Clearance ?

Its purpose is to allow some mechanical expansion and lengthening of the valve stem and push rods when the engine warms up.

This clearance is also known as valve lash.

Why Tappet Clearance is Given ? Why it is required ?

This small Tappet clearance gap is very essential between rocker arm and valve stem when the valve is in closed position.

  1. It is provided to allow thermal expansion of the valve spindle due to the heat of the engine (at working temperature ).
  2. It is used to ensure positive closing of the valve at all the working conditions and temperature.

The requirement of clearance is answered as, to maintain correct timing ( change in clearance will lead to an early or delayed opening of the inlet or exhaust valves ).

Note :- It needs to be adjusted for proper opening and closing of valves of the diesel engine.

Effect of Incorrect Tappet clearance of Exhaust Valve

When this clearance increases or decreases, there is an adverse effect on the timing of opening and closing of the valve.

  • If the clearance decreases- The valve will open early and close late.
  • If the clearance increases- The valve will open late and close early.

What will happen if Tappet clearance is Less :-

  1. Valve will open early and close lately.
  2. Air induced in combustion chamber for combustion through an inlet valve may leak out.So, less air will be available for combustion.
  3. Power of the engine will be reduced.
  4. Fuel consumption by engine will increase, and engine may become unbalanced and the exhaust temperature will be very high.
  5. The valve may remain open,in worst conditions,resulting in loss of compression pressure of the engine,burning of the exhaust valve and increase in turbocharger fouling.

What will happen if Tappet clearance is more :-

  1. The valve will open lately and close early.
  2. It reduces the maximum lift of the valve.
  3. Less heat energy to the turbocharger of engine, cause reduction in the scavenge air and hence power decrease.
  4. There is no proper removal of the exhaust gas.
  5. Hammering of the v/v stem cause damage to the valve stem.
  6. It causes noise and eventually damage the working surface.

How to check Tappet clearance ?

To check clearance, first sure that the unit you are going to check should be in Top Dead Centre ( TDC ).

The position of piston is At TDC checked by

  • The flywheel marking 
  • The fuel pump mark 
  • The checking the camshaft fuel 
  • The valve cam position.

If it is not at TDC then we use turning gear to make that unit at TDC. ( Note :- During turning make sure that indicator cock is open ).

Usually, there are 6-8 units in four stroke engines. So at the same time, the flywheel will show two units at TDC.

To make sure that particular unit is on the compression stroke and not at the power stroke, you can turn the push rods of both units by hands to check.

The one with free push rods is the one which is at compression stroke and the one with tight push rods is at power stroke.

So avoid the one which is at power stroke.

Now you need good filler gauges of different thickness.

Now gently try to push these filler gauges between the clearance of valve and rocker arm.

You may have to try different thickness. Just check in the manufacturer manual as for how much standard clearance should be.

Now you know that your clearance is less or more

How to Adjust Valve Tappet clearance / Adjustment Procedure

Prepare the following tools :-

Feeler gauge of different thicknesses.


  • The feeler gauge of 0.4 mm thick for the Suction/inlet valve.
  • The feeler gauge of 0.5 mm thick for the exhaust valve ( It depends on engine, it may be different for different engine. )

1. Take all the Safety Precautions.

2. Make sure the piston of the engine on the TDC if not then turn the flywheel.we can check it by

  • By using marking on the flywheel.
  • By using the fuel cam.
  • Push rod should be free. ( At this stage ,both the valve should be close i.e at the end of compression stroke)

3. Make sure that the engine has cooled down.

4. Loose the lock nut of the rocker arm.

5. According to manual of the engine use filler gauge between the gap of valve stem and rocker arm.

6. Now adjust the clearance between the valve stem & the rocker arm by tightening or loosening the nut below the lock nut.

Note:- When you move filler gauges while adjusting for particular clearance, you should be able to slide it smoothly (not too tight and not too loose ).

Let us understand How to adjust Tappet clearance by example:-

Tappet clearance
Valve head clearance adjusting the procedure

Prepare the following tools:-

  • The feeler gauge of 0.4 mm thick for the Suction valve.
  • The feeler gauge of 0.5 mm thick for the exhaust valve ( It depends on engine, it may be different for different engine. )

Make sure the piston is at TDC I.e compression stroke

Loose the locking nuts 2 & 4 and loosen the rocker arm by adjusting the screw 1 and the valve guard by adjusting the screw 3.

Push down the center of the valve guard from above (so that under this condition the clearance become zero).

Gradually tighten the valve guard to adjust the screw 3 so that the clearance becomes zero. After adjustment, lock the adjusting screw 3 not to turn, and tighten the lock nut 4 securely.

Put a feeler gauge into a gap between the rocker arm adjusting screw 1 and the valve guard.Gradually, tighten the rocker arm adjusting screw 1 until the clearance becomes narrow enough to enable the feeler gauge to be drawn out smoothly. Then, lock the adjusting screw 1 , and tighten the locking nut 2 .

After tightening the lock nut 2 securely, check that feeler gauge can be drawn out smoothly.

The valve tappet clearance is measured by

A.Vernier Calliper

B.Feeler gauge

C.Screw Pitch Gauge

D.Engineering Scale

Ans :- B

Valve Diagram of different diesel engine




Tappet clearance



Tappet clearance

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