Compression Testing

Compression testing is a way to determine engine health. We use differential compression testing in aviation because it is a more reliable method of testing engine compression than direct compression testing used in automotive applications. We will take a...


Some Safety Notes

Magnetos should be grounded and fuel should be shut off prior to performing a test in order to make certain that the engine will not accidentally start.

“Ensure the mags are not hot,” shares Noe. “Also, it’s good practice to disconnect all spark plug leads prior to doing a compression test.”

Another important safety note is to use two people whenever performing a compression test. “Two people are better than one when doing a compression test,” says Noe. “You always want to be on the ball.” Before pressurizing a cylinder, make sure that your co-worker has a good grip on the propeller.

Another couple of safety items are important to keep in mind. Since two of you will be working on the test together, one to operate the tester and the other to hold the propeller securely, clear communication is essential. Also, make sure that neither worker is standing in the path of the propeller just in case it slips away during the test. In addition, ensure the area around the propeller is free from any obstruction that could interfere with the test or cause damage should the propeller accidentally get away from the person holding it.

Noe shares another safety tip. “We installed unique couplings on our compression test adapter. They are different from the couplings on all our air hoses. That way, it is virtually impossible to connect it to un-regulated shop air.”

Interpreting Readings

Now you will take a reading on the tester. This is annotated as the actual amount of pressure in the cylinder over the regulated pressure. For example, if the cylinder gauge indicates 60 psi and the regulated pressure indicates 80 psi, then your compression is 60/80. In general, 60/80 is the minimum acceptable leak limit. Keep in mind if you are working on a TCM engine, you will use the limit determined using the Master Orifice Tool.

What Next?

Low compression test results don’t warrant immediate removal of the cylinder. A little more investigation is in order.

With your assistant holding the propeller, you can walk around the engine to listen for air leaks. This is a good way to determine where the problem is coming from. Exhaust valve leaks will be evident by air leakage at the exhaust. Intake valve leaks will be evident by air leaking at the intake. Leakage past the piston rings is evident by air leaks at the breather.

If you suspect a leak at the valves, you can stake the valves by gently tapping on the valve stems. This should dislodge any foreign debris present that could be causing a leak. Be sure to rotate the prop through before checking the compression again. If you don’t do this, you could hide a problem of egged or eccentric valve seats.

You also want to do a borescope inspection on any suspect cylinders. You can see the condition of the valves. This also gives you the opportunity to see the condition of the cylinder walls. You can see if there is excessive oil, or if there is corrosion.

The engine manufacturers have guidelines on inspection procedures to coincide with compression tests to help determine the health of the cylinder and the disposition should a reading be below the minimum limit. Be sure to refer to the manufacturer’s instructions for proper procedures and troubleshooting techniques and communicate clearly with the owner to best determine what action is required.

These have been a few tips on differential compression testing. For detailed inspection procedures and maintenance criteria, be sure to refer to the manufacturer’s instructions.

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