Low compression on one cylinder of a high-time O-320

The test applies regulated air through a calibrated orifice into the cylinder with the piston at top dead centre on the compression stroke, all plugs removed and the throttle open, and compares the gauge reading with the supply pressure. The reading is a measure of leakage, and the location of the leak is as important as the number.

• Air at the oil filler neck or crankcase breather means leakage past the piston rings. That is normal to some degree on any engine and increases as the bores wear.
• Air at the exhaust means an exhaust valve or seat; at the induction an inlet valve; at the cylinder head joint a gasket or a cracked head.
• Here the noise is at the oil filler, so the leak is past the rings.

52/80 is below the 60/80 figure the manufacturer's service instruction uses as the investigation threshold. That threshold does not mean 'remove the cylinder'. It means 'find out why and justify the decision'. The same instruction distinguishes ring leakage from valve leakage: leakage past the rings, with no other discrepancies, can be acceptable in service, whereas valve leakage is a removal item because a burnt valve can fail and destroy the cylinder and the engine.

Before drawing any conclusion, I would repeat the test. A single low reading can come from a carbon flake under a valve, a piston not exactly at top dead centre, or a cold engine, and a second test after a run often reads differently. Getting a stable, repeatable 52/80 with ring leakage is the finding to work with.

A return to service is defensible here, but only on evidence, not on 'it has always been like that'.

Evidence I want before signing:

1. Repeat compression test, ideally on a warm engine, consistent with the first result.
2. Borescope of the valves and seats on that cylinder, including a close look at the exhaust valve face and the seat for a wedge or a burnt area. Already done: clean.
3. No metal in the filter and the suction screen, and no abnormal ferrous material in the oil analysis.
4. Oil consumption stable at 1 quart in 5 hours, and the trend from the last few checks.
5. Crankcase breather clear, and no signs of blow-by such as heavy oil mist on the belly.
6. Run-up: normal mag drop on both magnetos, normal idle, normal oil pressure and temperature, no abnormal noise.

Given those, the manufacturer's guidance supports continued operation with ring leakage below 60/80. I would:

• Record the readings and the investigation in the engine log and the work order.
• Set a shorter recheck interval, for example compression and oil consumption at the next 50-hour check or sooner, and require the operator to report any change.
• Note the proximity to TBO (1,850 of 2,000 hours) and plan the engine's removal, or at least a cylinder work package, rather than waiting for a failure.

What would stop it: leakage heard at a valve rather than the rings, any visible valve distress on borescope, metal in the filter, a rising oil consumption trend, or a rough running engine. In those cases the cylinder comes off before the aircraft flies, and the owner's cross-country is cancelled.

Oil consumption is a trend indicator, not a single number. 1 quart in 5 hours is about 0.2 quarts per hour, which for a 160 hp engine is within normal expectations and not alarming on its own. What matters is whether it is stable.

• A slow, steady rise over hundreds of hours fits normal bore and ring wear, and it goes with the compression figure you found.
• A sudden rise on a stable engine points at something else: a stuck or broken oil control ring, a failed valve guide, a leaking seal or a crankcase pressurisation problem. A sudden rise plus oil mist on the belly and a dark exhaust is a stop item.
• Oil analysis is a screening tool, not an airworthiness finding by itself. Ferrous material suggests cylinder walls, rings, cam or lifters; aluminium or bronze suggests a bearing, a valve guide or a piston. A single elevated sample can be a one-off; two consecutive rising samples are a trend and need investigation.
• The filter is the hard evidence. Cut it open and look at what the magnet catches. Visible metal flakes or a sudden quantity of fine material is a grounding item, regardless of what the compression test and oil analysis say.

For this engine I would tell the owner: the compression reading and the oil consumption together say 'worn but serviceable, monitor closely', and the filter and the trend are what will decide whether that judgement holds. He should also know that at 1,850 of 2,000 hours he is buying an overhaul soon either way, and that planning it is cheaper than an unplanned removal.