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What is Tappet Clearance - How to Measure & Adjust Properly

Dimitris Tsapis
January 8, 2022
In this article we delve into the basics of understanding, measuring and adjusting tappet clearance to solve common engine issues on ships.

What is tappet clearance and how should you properly adjust it based on the engine’s needs? When talking about ship engines, this is one of the few issues that will come up sooner or later. Hence, it is an important skill for any engineer that will eventually troubleshoot engine problems.

In this article we delve into the basics of measuring, and adjusting tappet clearance to solve common engine issues on ships. After reading this post you should have a better idea of what tappet clearance is and why it’s important.


What is tappet clearance and when is it needed?

Tappet clearance, also known as valve clearance, is the small gap between the rocker arm and the top of the valve stem (Yoke). 

what is tappet clearance

Engineers can observe and adjust tappet clearance only when the valves are in a closed position and the engine is cooled at the compression stroke.

As for when you need to check tappet clearance, there are generally two indicators you need to keep in mind: 

  • First, as shown in the engine’s manual, the process should be taken after a certain number of running hours. These differ based on the engine model.
  • Second, if you notice irregularities coming from misaligned tappet clearance, like noise coming from the rocket arm. We discuss all issues resulting from incorrect tappet clearance in the following chapters.

Read also: What is Variable Injection Timing (VIT) - Theory & Practice

Purpose of tappet clearance

The main purpose of tappet clearance is to allow a small expansion gap of the valve stem and pushrods to accommodate the effects of a heated engine. 

Note that, if this isn’t done, the valve will not sit correctly as the engine heats up. As mentioned above, this could eventually result in power loss, irregularities in the engine, and other issues.

Why tappet clearance is necessary

The clearance between the valve stem and rocker arm is very important mainly for two reasons:

  • It accommodates the thermal expansion of the valve’s axis (spindle) that is caused by the heat of the ship’s engine when in operation.
  • It ensures the closing of the ship’s valve regardless of temperatures and working conditions.

In short, knowing what is tapper clearance and why it’s necessary will help you maintain the right timing through the delayed or early opening of the inlet. To do this, however, you need to know how to measure and adjust the open/close of the valves. So let’s briefly talk about this as well.

Tappet clearance measurement

To measure tappet clearance (a.k.a. perform a tappet clearance procedure) you will need to measure the increase or decrease of the clearance. Depending on that, you will notice changes in the timing of the opening and closing of the valve.

  • If the tappet 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 of time. With less clearance, we get a bigger contact area between the valve stem and rocker arm. In turn, this keeps the valve open for longer as the rocker arm pushes the valve down for a longer period of time.
  • If the tappet clearance increases, the valve will have a delayed opening and an early close. Since there is more clearance there is a smaller contact area between the valve stem and rocker arm. As such the valve remains open for a shorter period of time but remains closed for longer (which explains the delayed opening).

Symptoms of valve clearance problems

To understand whether the tappet clearance is wider or tighter than it should be, you can observe how the engine responds to it. Based on this, you will know if adjustments are needed.

When tapper clearance decreases:

  • The valve will open sooner and close later
  • You may observe air leaks in the combustion chamber, leading to less available air for proper combustion.
  • The power and performance of the engine will decrease
  • Fuel consumption of the engine will increase
  • The temperature of the exhaust will increase
  • In some scenarios, the valve could stay open for an extending period of time, causing decrease in the engine’s compression pressure. If this happens you may notice exhaust valve gets burn, and a spike in turbocharger fouling.

When tappet clearance increases:

  • The valve will open later and close sooner
  • The max lift of the valve will decrease
  • Reduction in scavenge air (power decrease) due to less heat energy being delivered to the engine’s turbocharger
  • Problematic removal of exhaust gas (not proper function)
  • Repeated collision with the rocker arm can cause damage to the valve stem
  • (Common tell) You may observe noise that, if left unchecked, can lead to damage on the working surface.

How to check tappet clearance - Practical applications

Let’s get to work now. How exactly can you check valve clearance? Before you start, make sure you review the manufacturer’s manual, as the explanation below is rather generic. Combining the two will usually lead to the best result.

So, how do we proceed?

To start, make sure that you take all the necessary safety precautions, make sure the engine is cooled, and check if the valves are closed.

Ensure that the piston is set in Top Dead Centre (TDC) by checking the following:

  • The marking of the flywheel
  • The marking on the fuel pump (& camshaft fuel)
  • Position of the valve cam
  • If the pushrods are not obstructed (as mentioned above, with both valves closed at this point, so the pushrod should be free)

If any of the above are not in TDC, don’t proceed to the next step. Instead, use the necessary turning gear when the indicator cock is open, to set them at TDC.

TDC explained

Four-stroke engines usually contain six to eight units. Therefore, the flywheel should now indicate two units set at TDC. 

If you want to check whether a unit is on the compression stroke instead of the power stroke, you can manually turn the pushrods of the TDC-indicated units. 

  • If the pushrod is free it’s at the compressions stroke
  • If the pushrod is tight it’s at the power stroke (avoid this one!)

At this point, you will have to find filler gauges, of multiple thickness sizes. Try to place them one by one between the valve clearance and rocker arm to find the ideal size. Usually, the manual of the manufacturers should speed up this process by indicating a range of sizes. Once you find the filler gauge that fits perfectly you will know if the distance between the valve stem and rocker arm is larger or smaller than it should be.

Read also: Engineer’s Guide to Marine Boiler Water Treatment & Testing

Valve tappet clearance adjustment

Once you have performed the check as demonstrated above, you can now proceed to a valve tappet adjustment. To do this, you will need once again need to use a filler gauge. In this case, the ideal size depends on the valve:

  • For the inlet valve choose a gauge with 0,4mm thickness
  • For the exhaust valve, choose a gauge with 0,5mm thickness (for this one review the manufacturer’s manual as well, as the size, often depends on the engine)

Similar to the course of action you took previously, ensure that all safety precautions are taking, the piston is set of TDC, and the engine is cooled down. 

Then, loosen up the lock nut of the rocker arm to place the gauges. Based on the manufacturer’s manual, place the recommended gauge between the valve stem and rocker arm, and then adjust the tappet clearance by tightening or loosening the nut under the lock nut. The filler gauge should not feel very tight in the clearance; instead, it should be able to slide through the gap.

To effectively measure tappet clearance during this stage, you can use one of the following:

  • Vernier caliper - Learn how to use it here
  • Screw pitch gauge - Learn how to use it here
  • Feeler gauge - Learn how to use it here
  • You can also use a typical engineer’s scale

Summing up

You should now know what is tappet clearance and what role it plays in the engine’s environment. The small gap between the rocker arm and valve stem is primarily focused on providing space for heat expansion, and can easily be adjusted based on the symptoms you might notice.

When measuring tappet clearance remember:

  • If the tappet clearance decreases, the valve will open sooner and close later.
  • If the tappet clearance increases, the valve will open later and close sooner.

When adjusting tappet clearance try to always use the manufacturer’s manual in conjunction to your knowledge, as every engine requires different attention and details.