Q: Over the last three years of annuals I have had three different mechanics. The mechanic does the compression check, calls out the cylinder number and I record the values. It appears to me that the mechanics might be confusing the cylinder numbers and I can’t find anything online that tells me in what sequence they are numbered.
My questions: What is the sequential numbers of the cylinders on my Lycoming O320E2D? What may be the primary cause of compression loss? What repair is required to correct the problem?
STEVE SARCHETT, via e-mail
A: This is one of those questions that more people would like to ask, but are hesitant to do so because they may get a few chuckles from their friends for asking such a basic question. Steve, I’m proud of you for bringing up the subject and I can guarantee that many pilots out there are also confused as to which cylinder is located where on their engines.
Let’s make this simpler by always viewing the engine from the pilot’s seat whenever we discuss the positioning of anything forward of the firewall. On the majority of Lycoming engines, the number 1 cylinder is the right front cylinder when viewed from the pilot’s seat. Moving rearward on that side of the engine is the number 3 cylinder. The numbers 5 and 7 cylinders complete that side up through the 8 cylinder series. Opposite the number 1 cylinder, on the left side of the engine from the pilot’s seat, is the number 2 cylinder. Moving rearward again, we’ll find the number 4, 6, or 8 cylinder, whatever the case may be.
Lycoming does have a few that are marked differently. These are some of the larger, more sophisticated engines, such as those found in the pressurized Piper Navajo, the Beechcraft 56 TC Baron and the Beech Duke. These were a completely different design for Lycoming because the camshaft was located on the bottom of the engine rather than on top. On these models, the number 1 cylinder is the left front cylinder and the number 2 cylinder is the right front.
Lycoming has a very good Service Instruction 1191A covering cylinder compression and I’d highly recommend every maintenance technician review a copy. (NOTE: If it were my aircraft, anyone working on it would have to show me that they had all of the manufacturers’ publications supporting that component, i.e., engine, airframe, etc.). It clearly explains the procedures, and the whys and wherefores of the test.
The compression check should always be conducted when the engine is at normal operating temperatures to get a more realistic reading. Doing it when the engine is cold does not provide reliable information. The test also must be conducted using the differential compression method rather than the direct reading compression method as used in the automotive industry. It’s also wise to use the same test equipment each time to eliminate any possible calibration issues.
The results we’re looking for should show no greater loss than 60/80 or no more than a 25% loss in any given cylinder. Readings such as 72/80 or 67/80 are OK. Should you get one cylinder that gives you a reading like 62/80 when all the other cylinders are reading 75/80, I’d fly the aircraft and retest in a few hours.
Always make a logbook entry of any compression checks even though they may be recorded on a shop work order. This makes it easy to review this portion of the engine’s health history.
Causes for loss of compression are varied and addressed in Service Instruction 1191A, but if we are seeing a differential compression reading of 58/80, we should be able to determine where the loss is occurring by just listening at a few different locations around the engine. For example, if we put our ear near the exhaust pipe and hear air hissing, then the loss is probably coming past the exhaust valve and seat. If we hear the air at the intake airbox (with the air filter removed), then we would suspect the intake valve and seat. If we can’t hear any appreciable air being passed there, then remove the oil filler cap. If you hear the air being lost in this location, it indicates the air is blowing past the piston rings. Once you’ve determined the location of the loss, you can proceed with the corrective action.
