What does a compression test tell you about an engine?
Updated · Wingman AeroCare, KFNL
Short answer: A compression test (on aircraft piston engines, a differential pressure test) measures how well each cylinder holds pressure, which shows how well the piston rings and the intake and exhaust valves are sealing. A low reading tells you a cylinder is leaking and, by listening for where the air escapes, roughly where. It does not by itself tell you whether the cylinder needs to come off; that decision should combine the reading with a borescope inspection, a re-test, and the engine's history.
How does a differential compression test work?
Unlike a car's compression test, which measures pressure while cranking, aircraft engines are checked with a differential pressure tester:
- The engine is warmed up, ideally by flying or a ground run, so the rings and valves seal as they do in operation.
- Each piston in turn is positioned at top dead center on the compression stroke, with both valves closed.
- Regulated shop air, normally 80 psi, is fed into the cylinder through a spark plug hole.
- A second gauge reads how much pressure the cylinder holds. A reading of "74/80" means 74 psi held with 80 psi applied.
The difference between the two gauges is the leakage. Part 43 Appendix D requires the annual and 100-hour inspection to check internal engine condition for cylinder compression, so every inspection includes this test.
What is a "passing" number?
It depends on the engine manufacturer:
- Continental uses a calibrated master orifice to establish the minimum acceptable reading for that specific tester, on that day, under those conditions. Its procedure is published in Continental Service Bulletin SB03-3 and Continental maintenance manuals. A cylinder at or above the master orifice reading is acceptable.
- Lycoming covers the procedure and its limits in Service Instruction SI 1191A, which also stresses comparing readings between cylinders and tracking them over time. FAA Advisory Circular AC 43.13-1B gives a general guideline that more than 25 percent leakage (a reading below about 60/80) calls for further investigation. Either way, a low reading is a reason to look closer, not to remove the cylinder automatically.
Because the tester, air supply, engine temperature, and technique all affect the number, a single reading matters less than the trend for the same cylinder over several inspections and whether it agrees with other evidence.
Where is the air leaking?
With pressure applied, the mechanic listens for escaping air:
| Air heard at | Likely leak source |
|---|---|
| Exhaust pipe | Exhaust valve not sealing |
| Air intake or induction | Intake valve not sealing |
| Oil filler or crankcase breather | Piston rings |
| Cylinder head or barrel (found with soapy water) | Possible cracked cylinder (rare) |
An exhaust valve leak is the one to take most seriously, because a poorly sealing exhaust valve can overheat and burn, and a burned valve can fail.
Does a low reading mean the cylinder must be replaced?
Not necessarily. Common causes of a falsely low reading include:
- Carbon or a small particle holding a valve slightly off its seat. Running the engine and re-testing, or carefully "tapping" the valve per manufacturer guidance, often clears it.
- Rings that have rotated so their gaps align. This can change after the engine runs.
- An engine that was tested cold or with the piston not exactly at top dead center.
The right next step after a low reading is usually:
- Re-test after running the engine.
- Borescope the cylinder to look at the exhaust valve, cylinder walls, and piston. See what a borescope inspection shows.
- Review trends: oil consumption, oil analysis, EGT and CHT data from an engine monitor, and past compression readings.
What can a compression test not tell you?
A cylinder can test well and still have problems:
- An exhaust valve beginning to burn may still seal when tested.
- Cam and lifter wear, bearing wear, and crankcase cracks do not show up.
- Corrosion on the cylinder walls may not yet affect sealing.
That is why compression is one data point among several. Good engine decisions come from the full picture.
How Wingman handles it
Wingman AeroCare works on Lycoming and Continental engines and performs cylinder and top-end work in-house, so a low compression reading leads to a real diagnosis rather than a reflexive cylinder swap. Compression readings and any related findings go on your online estimate with photos and video, labeled Airworthy, Recommended, or Optional, and you decide what happens next. Learn more about our piston engine maintenance or contact us.