What is Tappet Clearance ? How to check and Adjust Tappet Clearance ?


Below Figure showing Tappet Clearance in the Exhaust Valve of 4-Stroke Diesel Engine.

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

This Article will learn you :-

  1. What is it ?
  2. It’s purpose ?
  3. Why it is required in Exhaust valve ?
  4. Why it is important ?
  5. How it is Adjusted ?
  6. What will happen if clearance is less.
  7. What will happen if clearance is more
  8. Diagram

What is Tappet Clearance ?

It is a gap between the top of the stem of the valve and the rocker arm.


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.

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 ? Or Why it is required ?

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

  1. It provides room for thermal expansion of the valve spindle due to the heat of the engine.
  2. It is used to ensure positively closing of the vale

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

How to check clearance ? Or Tappet clearance adjustment 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. )

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

6.Use the feeler gauge to adjust the clearance of the suction valve as .35mm and the clearance of the exhaust valve as .45mm

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.

Effect of Incorrect clearance of Exhaust Valve

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 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.

The valve tappet clearance is measured by

A.Vernier Calliper

B.Feeler gauge

C.Screw Pitch Gauge

D.Engineering Scale

B.Feeler gauge





Tappet clearance



Tappet clearance


Tappet clearance