since 1940

Low compression in Continental or Lycoming aircraft engine

Although called a "compression" test it doesn't check for compression but checks for leakage past the sealing surfaces in the combustion chamber.

Possible sources of leakage (ranked from most to least common):

•  Exhaust valve
•  Piston rings
•  Intake valve
•  Crack or hole in cylinder head or piston.

To find out where it is leaking use the following tips:

•  listen in the exhaust for exhaust valve leakage.
•  listen in the crankcase oil filler for piston ring leakage or piston damage.
•  listen in the intake for intake valve leakage

Normal leakage numbers:

For Lycoming and Continental aircraft engines: Lycoming's will normally read in the 70 over 80's, Continental's will normally read in the high 60's over 80's. This means that a reading of 65 over 80 on a Lycoming is generally a low reading and, if past the piston rings. may result in increased oil being blown out the engine breather. While 65 over 80 on a Continental 520 engine is a good reading and would not normally result in any abnormal amounts of oil being blown out the engine breather.

There's a certain amount of normal or accepted ring leakage. Use the manufacturer's criteria for acceptable leakage past the rings. Lycoming and Continental differ in their recommendations. Check there latest publications for not only evaluating the results of the compression test but also the techniques for obtaining valid and repeatable readings.

Hydraulic Lifters Pumping Up

Valves can leak if the hydraulic lifters pump-up. This may result in the situation where the compression check is good, the engine starts to run OK but then a cylinder drops-off or the engine becomes rough. This happens when the hydraulic lifter pumps up and holds the valve open. An example of this happening is on the A-75, A-64 engine installed in an aircraft that originally used an oil cooler. Then the engine is removed and installed in an aircraft without an oil cooler. Now the engine drops two cylinders and runs rough because two hydraulic lifters on one side of the engine are pumping up and holding the valve open. The reason its doing this is that the 22130 sleeve (oil restrictor) was not removed from the crankcase. This oil restrictor diverts some of the oil thru the oil cooler. If you remove the oil cooler and don't remove the restrictor then pressure builds up on one side the the crankcase and the extra oil pressure causes the hydraulic lifters on that side of the engine to pump up and hold the valves open.

Larger Continental engines use a steel-insert in the top ring land to strengthen the land.

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.