Cylinder Compression Checks: What the Numbers Really Mean

Every piston aircraft owner eventually hears the words: “Compression on number three is a little low.” And almost immediately, the mental math starts. Top overhaul? Cylinder replacement? How many AMUs is this going to cost?

Take a breath. A compression number, in isolation, doesn’t tell you whether a cylinder is healthy or sick. It’s one data point in a larger picture — one that includes the master orifice, what a borescope reveals inside the combustion chamber, where the air is leaking, and how this cylinder has trended over time.

This post walks through the differential compression test the way an experienced GA owner should understand it. You don’t need to do the test yourself — that’s your A&P’s job — but understanding what your mechanic is doing (and what the numbers actually mean) makes you a better partner in maintenance decisions.

Why we do compression checks in the first place

The cylinder is where the engine’s combustion happens. For it to do its job, the combustion chamber has to seal: the piston rings have to seal against the cylinder walls, the intake and exhaust valves have to seal against their seats, and there shouldn’t be a crack anywhere letting pressure escape. When any of those seals breaks down, the engine loses power, burns oil, runs rough, or worse.

A compression check is a diagnostic tool to see how well the cylinder is sealing. On GA piston engines, the standard approach is the differential compression test, which is described in FAA Advisory Circular AC 43.13-1B (Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair) and in the engine manufacturer’s service documentation.

Compression checks are performed at every annual inspection, at every 100-hour inspection (for aircraft operating under that requirement), and whenever a cylinder issue is suspected based on roughness, oil consumption, or borescope findings.

How the differential compression test works

The differential compression test does not measure cranking compression like the test you might run on a car engine. Instead, it introduces a known pressure into the cylinder and measures how much of that pressure leaks out.

Here’s the basic procedure:

1. The engine is warmed up to operating temperature. (A cold engine produces unreliable readings.)

2. The propeller is rotated until the piston in the cylinder being tested is at top dead center on the compression stroke — both valves closed.

3. A specialized differential compression tester is connected to the spark plug hole. It has two pressure gauges: one shows the regulated input pressure (typically 80 psi), and the other shows the actual pressure that holds inside the cylinder.

4. Compressed air is introduced. The tester reads as “X over 80” — for example, 74/80 means 74 psi held in the cylinder while 80 psi was applied at the input. The difference (6 psi) leaked out.

5. While air is flowing, the mechanic listens at the exhaust pipe, the intake (carburetor or induction system), the oil filler/crankcase breather, and the adjacent cylinder spark plug holes. Where you hear the leak tells you what is leaking.

The leak source matters more than the number. Air hissing from the exhaust means the exhaust valve isn’t sealing. Hissing from the intake means the intake valve. Air bubbling out of the oil filler means it’s getting past the rings into the crankcase. Each of those tells a different story about cylinder health.

The “60/80 rule” — and why it’s not really a rule

For decades, the conventional rule of thumb in GA was: anything above 60/80 is fine, anything below is a problem. You’ll still hear this from pilots, owners, and even some mechanics. It’s embedded in shop talk.

But this rule of thumb is not what current Continental guidance actually says. Continental’s published differential compression test procedure (Service Bulletin SB03-3, in its current revision) directs mechanics to compare cylinder readings against a calibrated master orifice rather than against a fixed number. The master orifice is a precisely sized leak built into a calibration tool; it establishes the lowest acceptable cylinder reading for the specific tester being used. If a cylinder reads at or above the master orifice value, it’s acceptable. If it reads below, it needs further investigation.

Why does this matter? Because differential compression testers vary in their input air flow capacity, and the “leakage” reading is influenced by the tester itself. Comparing every cylinder against a generic 60/80 threshold ignores the calibration of the specific tool being used. The master orifice approach corrects for that.

Lycoming’s service publications similarly emphasize that compression readings should be interpreted in context, not against a single hard-line number, and that borescope inspection is an essential part of the evaluation.

Why a borescope inspection should always come with the numbers

Modern guidance from both Continental and Lycoming reflects a clear shift: a numerical compression reading by itself is not enough to judge cylinder health. A borescope inspection — physically looking inside the combustion chamber with a small camera — should accompany the compression check whenever practical.

The borescope shows things the gauges can’t:

Valve condition. Exhaust valves can develop hot spots, irregular wear patterns, or asymmetric coloration long before compression drops. A “green” reading with a visibly burned exhaust valve is a cylinder on its way to a problem.

Cylinder wall condition. Score marks, glazing, or unusual wear patterns on the cylinder walls explain why ring sealing might be marginal — and whether the cylinder is salvageable.

Piston crown condition. Cracks, erosion, or detonation damage often show up on the piston top before they show up in the numbers.

Spark plug condition (indirectly). When the borescope shot reveals fouling, lead deposits, or unusual color, it points to operational issues (mixture management, plug heat range) that can be addressed without ever pulling a cylinder.

Many shops that work heavily on piston GA engines now consider the borescope mandatory at every compression check. It’s a reasonable thing for an owner to ask for — and to ask to see the images, if you’re curious.

What causes low compression readings

If the number does come back low, the cause is usually one of three things:

Valve leakage. Air escaping past an exhaust or intake valve is the most common finding. Sometimes a one-time low reading is caused by a piece of carbon temporarily holding the valve off its seat. The classic remedy — sometimes called “staking the valve” or “running the engine and rechecking” — can resolve a borderline reading if a borescope shows the valve face is otherwise healthy. Your A&P will make that judgment call.

Ring leakage. Air leaking past the piston rings into the crankcase, audible at the oil filler. New cylinders go through a break-in period where ring seating gradually improves; very old cylinders eventually wear past the point where the rings can seal well. Oil consumption usually trends up alongside ring wear.

Cracks or damage. A cracked cylinder head, a cracked piston, or a damaged valve will produce a low reading and is not something that gets “run in.” The borescope is essential for catching these.

One low reading on one cylinder, with the rest of the engine running well and no other symptoms, is usually a watch item rather than an emergency. Multiple low readings, or a low reading combined with rising oil consumption, rough running, or visible damage on the borescope, is a more serious finding.

Trends are more useful than snapshots

If you only ever see a single compression reading from a single inspection, you’re missing the most valuable information: how that cylinder has changed over time.

A cylinder that has been reading 74/80 consistently for the last three annuals and just came in at 72/80 is doing fine. A cylinder that read 76/80 last year, 72/80 at the recent 100-hour, and now 66/80 at annual is telling a different story — even though all three numbers are in the “acceptable” zone. The slope matters more than the absolute value.

This is exactly the kind of data that gets lost when maintenance records live in three different paper logbooks and a shoebox of receipts. The trend isn’t visible until someone sits down and reconstructs it. A maintenance tracker that records compression readings cylinder-by-cylinder across years makes the slope obvious at a glance.

Track compression cylinder-by-cylinder, every time. Don’t just record “compressions OK.” Log the actual reading on each cylinder at each inspection, along with the master orifice value if your shop documents it. After two or three inspections you’ll have a real trend — and you’ll see weakness developing long before it forces a decision.

Questions worth asking your mechanic

Useful questions during the compression check:

• What did the master orifice read on your tester today?
• Was the engine fully warmed up before the test?
• For any low readings — where was the air escaping?
• Did you do a borescope inspection of each cylinder? Can I see the images?
• How does this reading compare to last annual or the last 100-hour?
• If a cylinder is borderline, what’s the recommended next step — recheck, monitor, or further investigation?
• Are there any operational changes (mixture leaning practice, run-up procedure, oil type) that could help going forward?

None of these questions are gotchas. They’re the same questions a thoughtful A&P is already considering. Asking them out loud just gets you both on the same page.

What good and bad practice look like

Less useful approach:

• Treats “60/80” as a hard pass/fail line
• Records only the number, not the leak source
• Skips the borescope
• Doesn’t document the master orifice value
• Reacts to a single low reading as if it’s an emergency
• Loses the year-over-year trend in paper records

Better approach:

• Compares each cylinder to the master orifice on the actual tester used
• Notes the number, the leak source, and the engine temperature
• Pairs every compression check with a borescope inspection
• Records the master orifice value alongside the readings
• Treats one borderline reading as a watch item, not a verdict
• Tracks each cylinder’s readings over time to see the slope

The bottom line

Compression checks are an important diagnostic tool, but they are not a verdict. A single number means very little without three other pieces of context: the master orifice value of the tester being used, the location of the leak, and the trend over time. Add a borescope inspection and you have a real picture of cylinder health.

The next time you hear “compression on number three came in a little low,” you’ll know what to ask next — and you’ll be in a much better position to make a sound decision about your engine.

And if the answer turns out to be that the cylinder really does need work, that’s information you would have gotten to one way or the other. Better to get there with the full story than with one scary number out of context.

This is one of the core problems TachMinder was built to solve. When you log your inspections, oil analyses, and engine health data, the trends become visible — cylinder by cylinder, year over year — before they become problems.