The condition of the engine cylinders is important to performance of the engine. At Little Flyers we use two types of cylinder compression checks, differential and direct.

Differential Compression Check

We use the differential compression cylinder test to check the sealing of the piston, rings, and cylinder . In our shop, we test both the top (end of the compression stroke at the top dead center position) and the bottom (just before the exhaust valve opens at the end of the power stroke) of the piston movement. We use special differential compression test gauges for this cylinder check. An adapter is installed in either the top or bottom spark plug hole (which ever is more accessible), and the compression test equipment is then attached to the adapter via a flexible hose. Regulated air pressure (normally 80 PSI indicated by gauge ported to regulator) is sent through the following: a calibrated orifice (restrictor), a shut off valve (our version), a second indicating gauge, a flexible hose, a cylinder adapter, and the cylinder. As the air enters the cylinder, air pressure is indicated on the second pressure gauge. The difference between the regulator indicator gauge and the second gauge is the differential pressure. The drop in air pressure is due to the restriction of air flow limited by the orifice. When the leak of the cylinder is greater than that of the orifice, a lower pressure (leak) is indicated on the second gauge. On a less than perfect cylinder, leaks are due to excessive air passing by the rings, valves, and piston. Engine manufactures publish leak down limits according to what they consider an acceptable leakage rate. The compression gages / orifice can be verified for accuracy by connecting the compression tester to a master test orifice which indicates the maximum allowable leakage rate (drop in pressure). Our differential compression tester indicates 65PSI when attached to this orifice. The specifications for the orifice size in the compression tester can be found in the FAA's publication Advisory Circular 43.13-1B paragraph 8-14b. For an engine cylinder having less than a 5.00-inch bore; 0.040-inch orifice diameter; .250 inch long; and a 60-degree approach angle. For an engine cylinder with 5.00 inch bore and over: 0.060 inch orifice diameter, .250 inch long, 60 degree approach angle. The publication specifies a reading of 60/80 with this internal orifice.

Direct Compression Check

The direct compression check (like the automotive version) verifies the actual pump action of the cylinder. A pressure gauge is connected to each cylinder which incorporates a check valve at the cylinder end to retain the pumping pressure. The engine is turned with the starter (with the ignition system off) until the gauges stop increasing in pressure. The indicated pressures on the gauges are the direct compression pressures. The direct compression test indicates two things: the actual pumping pressure of the cylinder and the relationship of the pressure with respect to the other cylinders. Any spread of more than 20% on the readings is suspect.

Combined results

Together the cylinder checks give us a good indication of the cylinders condition. Often a cylinder problem can be detected about 100 to 150 hours in advance of when the cylinder may need repair or replacement.

The Compression Check as Performed by Little Flyers

Preparations

The engine is ground operated until cylinder and oil temperatures stabilize. A ground run is necessary due to the difference in co-efficient of expansion of aluminum (piston) and steel (cylinder), otherwise the compression test may be unreliable. Some cylinders are manufactured with the top bore of the cylinder slightly smaller, it is called choke. Cylinders are choked because the top of the cylinder operates at a higher temperature. When the cylinder expands at operating temperature the bore becomes the same dimension from top to bottom. The engine warm up is usually accomplished as part of the pre-inspection run-up; during this time crankcase pressures are taken. High and increasing case pressure readings will be manifested long before any decay in compression test readings are noticed. Tracking case pressures over a period of time will give a savvy technician additional clues as to cylinder / piston / ring conditions. See TCM SB 89-9 for Continental specifications. We also verify grounding of BOTH magnetos with ignition switch(es) “OFF” for personal safety and to prevent inadvertent engine firing. The unexpected firing could damage our direct compression test equipment. Compression checks are conducted as soon as possible after shut down to maintain cylinder / piston / ring operating temperature and clearances. Cowling, selected cooling baffles, and intercooler are removed as required; the most accessible spark plug is removed from each cylinder. We quiet the shop environment as much as possible as to detect additional fault clues by sound.

Differential Compression

Differential compression adapters are installed into each open spark plug hole. Using one adapter will result in progressive cooler cylinders; extra time involved in removing and replacing in other cylinders results in less reliable data. Oil filler cap is removed from filler neck to observe residual vapors and hear possible ring leakage.

The differential compression tester is connected to shop air and to one of the previously installed compression adapters. An assistant turns the prop in the normal direction of rotation until he feels the resistance of compression. the piston is stopped just short of it’s full travel. The technician turns on the compression tester air valve while assistant holds prop in position. The technician adjusts the “in” gauge to indicate 80 PSI with the air regulator. The “out” gauge (second) on the down streamside of the calibrated orifice is read and recorded.

Now, the assistant continues to turn the prop in normal direction of rotation always keeping some air pressure against piston and rings; the assistant stops turning just before exhaust valve opens. The “out” gauge is then read and recorded. The test proceeds on to the remaining cylinders. Compression readings are taken at both top and bottom of the piston stroke for more data.

We are alert for following when taking differential readings:

1. Ring snap on applying air pressure that may indicate worn rings and piston ring lands.

2. Air noise / vapors from oil filler neck indicating leakage past piston rings.

3. Air noise from induction system opening indicating intake valve leakage.

4. Air noise from exhaust system opening(s) indicating exhaust valve leakage.

5. Any suspected leakage between cylinder head and barrel is checked with a soap solution (that will bubble if seal is defective).

6. Air leakage between head and barrel cooling fins, with piston toward bottom of stroke indicates cylinder barrel cracks.

7. A differential of 6 PSI or more between top and bottom usually indicates cylinder bore / ring problems.

We recheck any low cylinder, we DO NOT condemn cylinders (unless cracked) on the basis of only one test. Skill and experience are required to properly interpret excess leakage. Static leakage normally requires immediate repairs / replacement before continuing in service. If all readings are low, we check our compression tester with Burrough’s Test Orifice TCM pn: 646953.

The differential test equipment is then removed from the engine.

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Direct Compression

We install direct compression adapters and gauges on all cylinders as single cylinder checks do not reveal as much useful data. Magnetos are grounded to prevent cylinder firing during direct compression test.

Throttle is open, mixture control is selected to “idle cut off”.

The engine is cranked using auxiliary power if required) until gauges cease to rise in pressure (about 12 revolutions). Direct readings are then recorded.

An engine with good compression on all cylinders will crank with consistent rhythm and sound.

Evaluating Readings

Readings will vary with test equipment, techniques, engine temperatures and type / grade of oil. Subsequent tests may result in varied readings. Rocking the prop can usually increase differential readings. Lycoming engines generally seal better and have higher readings than Continental. Continental engines with steel inserted piston ring lands generally exhibit more normal ring leakage. Nearly equal top and bottom differential figures reflect desirable bores. Sticking / stuck rings can cause erratic readings.

Direct readings should be proportional to compression ratios. Expect high compression engines to be 140-170 PSI, 80 Octane engines 120-140 PSI and Turbo-charged engines 100-130 PSI. Engines with equal direct readings tend to run smooth. A 20% spread between low and high cylinders is considered normal.

Excess spreads may be caused by a mixture of high and low compression pistons, mixture of standard and oversize bores. Excess build-up of carbon from rich mixture or excess upper cylinder oil will result in higher than normal readings.

A low direct reading with good differential reading may indicate a lower compression piston, sticky valve, or severely worn cam lobe(s). A higher than normal direct compression reading (with normal differential reading) may indicate a high compression piston installed or excess head welding that was not properly profiled. Oversize bores may indicate slightly higher pressures.

Cylinders having low readings with both differential and direct compression should be carefully checked out before returning to service.

A direct compression test (even cold) on a cylinder replacement will alert you to possible wrong piston configuration and excess remaining head welding material.

Addition information can be found in AC43.13-1A Chapter 14 paragraph 692, TCM SB 84-15 and Lycoming SI 1119A.

Submitted 8/28/00 by Roger A. Stern A&P / IA

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