Cylinders condition and compression tests

Cylinders condition and compression tests

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(A warning to aircraft owners against pulling jugs upon a mechanic’s recommendation)

The compression test is a procedure that might have been born during the wright Brothers adventure. It’s an old test that doesn’t really tell is much about the health of our cylinders. 

Yet somehow it’s remained the gold standard for A&P’s. It belongs in the dustbin of history. 

Knowledge from 50 years ago (or lack thereof, now knowing what we know today) told us to remove a cylinder when compressions yielded below 60/80, and even now 40/80. Ask engine manufacturers today, and they’ll tell you stories of testing out engines with compressions in the teens, and still making proper horsepower. Compressions DO NOT necessarily equate to an engine making proper HP or not. If it wasn’t for the FAA requiring a “cylinder leaking” test at annuals, engine manufactures would do away with the requirement all together!

Continental’s latest Service Manual “Manual M-0”, details the correct procedure for properly performing a cylinder leakage test, aka “Compression Check/Test”. For cylinders under 5″ diameter barrel bore, a differential compression gauge w/ a fixed master orifice of .040″ is required. For engines with cylinder bores 5″ or greater, a master orifice of .060″ is required by the latest Service Manual. If your mechanic tells you something different, show them the Manual M-0. Not using the required gauge will give the mechanic the wrong information, and lead him or her to make the wrong, and ill-informed recommendation to you as the aircraft owner.

In the event a compression check shows a “failing” compression reading, DON’T REMOVE THE CYLINDER JUST YET! The fun has just begun! Continental provides a very detailed set of tasks to follow, step by step, to inspect the cylinder by other means.

Any competent shop will posses a HIGH QUALITY borescope, such as the one that we use from Vividia. Wingfield Aviation uses the “Vividia Ablescope UV-400 Rigid w/ LED Lights”, and we view the images on a large portable flat-screen TV in our shop.

The face of the exhaust valve is supposed to be symmetrical in color with no green spots. Any competent A&P should perform this test to detect an up and coming exhaust failure. You should consider talking to your A&P about adding this inspection at every annual and/or spark plug service. It’s quick and easy and may catch a catastrophic failure before it happens. 

If a cylinder’s compression check reveals a less than desired result of less than 40, the next step is to visually inspect the cylinder. Key things to look for are unusual wear marks such as vertical scoring of the barrel, which can be caused by improper lubrication at installation and/or rings not being filed properly if needed. However, modern day machining produces rings that rarely require filing. Another important item to look at is the condition of the exhaust valve face, coloration, burn pattern, and condition of the valve seat.

Ideally, a hot red concentric center, or white ring on the valve’s face is a sure sign of great valve health (Pic #1 & #2). With the prop cycled on the exhaust stroke, use the borescope to inspect the condition of the valve seat. In the event the valve needs to be “lapped”, know that this task can be performed with the cylinder still in place on the engine, and does not have to be removed. Cylinder removal, or even worse, removal of multiple cylinders is a sure invitation for “MIF”, aka Maintenance Induced Failure. (I’ll spare you of the horror stories that have come from removing cylinders that don’t need to be removed, and then shops not reassembling engine components back properly after replacing perfectly healthy cylinders.)

After visual inspection of the interior of the cylinders, Continental’s Manual M-0 REQUIRES (not recommends) a minimum 45 minute test flight at cruise power. Recommendation by a mechanic that says a “run up on the ground is sufficient” should be sent packing. This is clear evidence that such an individual is not following the manufacturer’s Service Manual, which is the standard and “law” in which to perform work in accordance with.

After a 45 minute test flight at cruise power, a recheck of compressions is then required. We’ve heard plenty of stories where cold compressions started off in the 20’s & 30’s, and then rechecked after the REQUIRED 45 minute test flight back up into the 70’s. Again, compressions don’t necessarily equate to engine health.

In the event a shop says they will “ground your aircraft” based on failed compression readings, remind them that the FAR’s don’t grant them that right or ability to ground an aircraft. Demand your aircraft back, and take it to a shop that is knowledgeable enough to be an advocate for you, the plane owner!

Remember, more times than not, the test is flawed (due to the tester), and not the cylinder.

Wingfield Aviation would be happy to perform a borescope visible inspection of all of your cylinders & show you what we see!