Compression Testing

April 1990

There are two accepted methods of testing the compression in a cylinder of an internal combustion engine. One is the "direct" method, generally used by auto mechanics on auto engines. This method uses a pressure gauge which is connected directly to the spark plug hole and the engine is than turned over with the starter or the engine and is run at idle. The peak pressure is read directly from the gauge. This method works but the results are not as precise as the method know as "differential compression" testing. This method is what is normally used in aircraft engines and requires the use of a tester consisting of two separate pressure gauges, a pressure regulator, a calibrated restrictor orifice, and an on/off valve. (See schematic) A source of compressed air (a compressor with a storage tank capable of a minimum of 100 psi) is required to perform the test. When you buy your differential compression tester, be sure it has a restrictor orifice of .040" (assuming your engine has less that 1000 cubic inches of displacement. An 0-235 has 235 cubic inches, and 0-360 has 361 cubic inches).

Your can find several suppliers of good reliable differential compression testers at Aircraft Spruce or Wicks, or even "Trade-A-Plane". Continental, Lycoming and the FAA all agree that the compression test should be performed with the engine hot. This assures that you get optimum piston ring and valve seating. In any event, you should try always to use exactly the same procedure with each cylinder and each time you check your compression, if your testing is to give meaningful and comparable results. Careful and regular compression testing say, every 100 hours, can be one of the best, most cost effective preventive maintenance procedures. It is very important that accurate records are kept of which compression reading was for which cylinder! You can read the number of each cylinder at the base of the cylinder.

Note that Lycomings and Continentals use a different numbering system. Remove the top spark plug from each cylinder and, for safety, remove each ignition lead from the bottom plugs. Rotate the prop by hand, in the normal direction of rotation (anti-clockwise for an American engine) until one of the cylinders comes up on compression. You can determine this by placing your thumb over the spark plug hole and feeling for a pressure buildup. Now, install the adapter (normally supplied with the compression tester) in the spark plug hole of the cylinder to be tested. Be certain that the air shutoff valve on the tester is off and connect the differential compression tester. CAUTION: Be absolutely certain the shutoff valve is closed and that you have a firm grip on the tip of one blade of the prop before connecting the system to your source of compressed air. You will now have to find top dead center on the cylinder being tested. The easiest way to do this is to adjust the pressure regulator to about 20 psi and open the air shutoff valve.

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Carefully rotate the prop in the normal direction of rotation against the 20 psi pressure until you feel a "flat spot" or rapid loss of turning resistance. If you go too fast, back up beyond top dead center and try again. It is critical that you reach TDC with the prop turning in the normal direction of rotation, not while backing the prop up since this would unseat the piston rings. The piston rings must be at the bottom of their lands in the piston with the piston at the top of its travel. (Click on image to enlarge) Now, be certain you have the prop tip securely held. This is a good time to have a second person to help you. The air shutoff valve should be open and the pressure regulator adjusted to show exactly 80 psi on the pressure regulator gauge ' Use caution because if you let the prop move in either direction beyond TDC, it will rapidly begin to rotate and it could beat the tar out of the unfortunate person who should have been holding it securely!

Now, gently move the prop tip back and forth, just a tiny amount. Watch the cylinder pressure gauge and take a reading from it at its peak steady pressure. Again, this will be while moving the prop in the normal direction of rotation. Be certain that the regulator pressure gauge is holding precisely 80 psi. You should have a differential pressure reading of between 60 and 78 over 80. Repeat this test as consistently as possible on all cylinders. You should now have a series of numbers something like this, depending on the condition of the engine: 76/80, 74/80, 73/80 and 75/80. These numbers, hopefully, will be fairly close to each other in magnitude. What are the limits? What constitutes a bad (too low) cylinder? It is generally accepted that a cylinder reading below 60/80 would require removal from service.

There is no rule or law that says this is the case. In fact, the FAA as well as the two engine manufacturers have no such requirement. You should probably continue to operate the engine and check the compression every 20 hours or so if the compression is 50/80 or above. Before you remove any cylinder, it would be a good idea to borescope the cylinder. That is, to look inside through a spark plug hole using a light and a special optical device known as a borescope. A single compression test does not necessarily mean anything. A single oil analysis also means very little. No single diagnostic test should ever be used to decide the health of your engine. The key is to do these tests regularly and keep good records of what you see.

Compare each test and make your decision based on several tests conducted over a reasonable period of time. If you have an abnormally low cylinder, you should start the engine and run it on the ground or even fly around the pattern once. Test it again. If it is still low, use a length of garden hose as a "stethoscope" and listen at the exhaust of the ailing cylinder. If you hear a hissing escape of compressed air here, you have an exhaust valve that is not seating. Similarly, listen carefully with the "stethoscope" at the carb or intake airbox. A hissing sound here would indicate leakage under the intake valve. If neither of these areas is leaking significantly, listen at the breather or oil dipstick/filler tube. A leak in this area is indicative of ring blow-by. This could be ring wear, barrel wear or scoring, or all the ring gaps may be lined up.

Hissing between cylinder cooling fins is bad news, possibly a cracked cylinder. Valve leakage is the most commonly found cause of a low cylinder. The differential compression test has its limitations but it still remains one of the best, most cost effective preventive maintenance procedures available to the builder/flyer. The method described here is simple and it works. Done every 100 hours regularly, you could save big bucks in the long run. If you would like to learn more about this procedure and many other cost saving tips for keeping your engine in good shape, you could not do better than to obtain a copy of "Top End" from the Light Plane Maintenance Library.

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