What is Tappet Clearance? Why is it Required and How measure it? | Valve Clearance

What is Tappet Clearance?

Tappet Clearance is the distance between the top of the valve stem and the rocker arm of the marine engine. It is also known as valve clearance.

Tappet clearance, also known as valve clearance, is the distance between the top of the valve stem and the bottom of the rocker arm or cam follower when the valve is in the closed position.

Valve clearance is important to ensure that the valve can fully close and seal against the valve seat, allowing the engine to operate properly. If the clearance is too large or too small, it can lead to problems such as noisy valve operation, reduced power, and even engine damage. Tappet clearance is typically adjusted by turning an adjustment screw on the rocker arm or cam follower.

Why is Tappet Clearance required?

Purpose of Tappet Clearance: The main purpose of Tappet clearance is to allow for the thermal expansion of the valve spindle length at working temperature and avoid any thermal stress.

It also ensures the positive closing of the valve at all working conditions and temperatures.

If the tappet clearance (valve clearance) decreases, the valve will open sooner and close later. This is because the locker arm stays connected to the valve for a longer period. With less clearance, we get a bigger contact area between the valve stem and the rocker arm. In turn, this keeps the valve open for longer as the rocker arm pushes the valve down for a longer period.

If the tappet clearance increases, the valve will have a delayed opening and an early closing. Since there is more clearance, there is a smaller contact area between the valve stem and the rocker arm. As such, the valve remains open for a shorter period of time but remains closed for longer (which explains the delayed opening).

It is a critical specification in the design and operation of internal combustion engines. The clearance is the amount of space between the valve stem and the rocker arm or cam follower that actuates the valve.

In an overhead valve (OHV) engine, the valve stem extends up through the cylinder head and is opened by a camshaft that pushes against a rocker arm or cam follower. In an overhead cam (OHC) engine, the camshaft is located above the cylinder head and directly actuates the valve through a cam follower or a bucket tappet.

The tappet clearance is important because it allows for proper valve operation. When the valve is closed, the clearance ensures that the valve fully seats against the valve seat, preventing any leakage of gases in or out of the combustion chamber. If the clearance is too small, the valve may not fully close, resulting in a loss of compression and reduced engine performance. If the clearance is too large, the valve may not fully open, reducing power and efficiency.

Tappet clearance is typically adjusted by turning an adjustment screw located on the rocker arm or cam follower. The adjustment screw changes the position of the rocker arm or cam follower relative to the valve stem, which in turn changes the clearance. Manufacturers provide a recommended clearance specification that must be followed to ensure proper engine operation.

How to measure the Tappet Clearance?

The amount of clearance should be monitored, as excessive and minimal clearance both hurt the normal functioning of the engine, as the change in Tappet Clearance will lead to an early or delayed opening of the inlet or exhaust valves.

Steps to measure the Tappet Clearance:

Before starting, take all the necessary safety precautions, such as wearing PPE, cooling the engine, and checking that the valves are closed.

1. Ensure that the turning gear sets the piston in Top Dead Centre (TDC).
2. Tappet Clearance is measured using a feeler gauge of multiple thicknesses.
3. To find the ideal size, try placing them one by one between the valve clearance and the rocker arm.
4. Once you find the filler gauge that fits perfectly, you will know if the distance between the valve stem and the rocker arm is larger or smaller than it should be.

Some common doubts or questions regarding tappet clearance may include:

What is the tappet clearance for the inlet and exhaust valve?

Answer: The value of the tappet clearance for the inlet and exhaust valve is the same for Daihatsu Engine, but it is different for Yanmar Engine. The value of tappet clearance depends upon the manufacturer, so it is advised to check the manual before adjusting the tappet clearance.

What is the clearance range in MM for the tappet?

Answer: The Valve clearance should be between 0.25 and 0.35 mm for the intake valve, and between 0.45 and 0.55 mm for the exhaust valve.

But for some engines, it can be equal for both the inlet and the exhaust valve.

How often should tappet clearance be checked?

Answer: The frequency of tappet clearance checks and adjustments varies depending on the specific engine and manufacturer recommendations. Generally, it is recommended to check and adjust the clearance periodically, such as during routine engine maintenance.

How to adjust the tappet clearance on a marine diesel engine?

To perform this adjustment, you will need specific feeler gauges: a 0.3 mm feeler gauge for the intake valve and a 0.6 mm feeler gauge for the exhaust valve.

The procedure for adjusting the valve head clearance is as follows:

1. Turn the flywheel and move the relevant cylinder to the top dead center of the compression stroke. A notice advises being careful not to move to the top dead center of the valve overlap.
2. Loosen the locknuts 1 and 3.
3. Loosen the rocker arm adjusting screw 2.
4. Loosen the valve bridge adjusting screw 4.
5. Press down the center of the valve bridge. Hold it and gradually tighten the valve bridge adjusting screw to lock it to prevent movement.
6. Tighten the locknut securely.
7. Put the rocker arm adjusting screw and the feeler gauge into the valve bridge clearance A.
8. Gradually tighten the rocker arm adjusting screw. Make sure there is enough space to easily remove the clearance gauge.
9. Tighten the locknut.
10. Check if the clearance gauge can be removed easily.

A notice also states that the valve head clearance of an intake valve and an exhaust valve are different, and you should make sure that you use the correct clearance.

What tool is used to measure valve clearance?

Answer: A feeler gauge is used to measure Valve Clearance or tappet Clearance also with other tool such a screw driver and spanner to adjust the Valve Clearance or tappet Clearance.

What tools are needed to adjust the tappet clearance?

Answer: The tools needed to adjust tappet clearance will depend on the specific engine design, but typically include a feeler gauge to measure the clearance and a wrench or socket to loosen and tighten the adjustment screw.

What happens if the tappet clearance is not adjusted correctly?

Answer: If the tappet clearance is not adjusted correctly, it can lead to problems such as noisy valve operation, reduced power, and even engine damage. If the clearance is too small, the valve may not fully close, resulting in a loss of compression and reduced engine performance. If the clearance is too large, the valve may not fully open, reducing power and efficiency.

Can tappet clearance affect fuel economy?

Answer: Yes, tappet clearance can affect fuel economy. If the clearance is too small or too large, it can cause the engine to run inefficiently, reducing fuel economy. Properly adjusted tappet clearance ensures optimal engine performance and fuel efficiency.

Can tappet clearance be adjusted by a non-professional?

Answer: Tappet clearance can be adjusted by a non-professional with the proper tools and knowledge of engine mechanics. However, it is recommended to have it done by a professional mechanic to ensure it is done correctly and to prevent potential engine damage.