Cylinder Differential Compression Testing: Subject to interpretation

Cylinder Differential Compression Testing Subject to Interpretation Teledyne Continental Motors May 2000 In its simplest terms, differential compression testing is a tool that maintenance personnel use to check the health of an aircraft...

Secondly, the determination must be made as to the source of the leakage: static or dynamic. The static leak sources are the valve seats, spark plug port seals, and cracks in the cylinder head or barrel or cylinder to barrel joint seal. Dynamic leakage occurs by the rings.

All static leaks are unacceptable and must be corrected. This, however, does not automatically translate to cylinder removal and repair. The key is to understand that the cylinder environment is a very dynamic one. It is not uncommon for small bits of combustion products to occasionally lodge under a valve seat. To examine this possibility, the cylinder should be "staked," a simple process of tapping the valve stem lightly with a plastic mallet while the cylinder is under pressure. The slight movement may dislodge the particle and allow a more effective seal to be established. You should verify that this test has been done, especially if you have not been noticing the engine roughness which is characteristic of a leaking valve seat.

Differential compression inspection checklist for Teledyne Continental Motors engines

Make sure you obtain possession of TCM Service Bulletin M84-15 which defines the procedures for obtaining accurate differential compression tests. This bulletin must be followed with discipline to obtain accurate test results.

Use the TCM Master Orifice Tool P/N 646953A and obtain the Master Orifice Reading. Test results are unreliable unless the Master Orifice Tool is utilized.

Properly identify any leakage. Is it static (valves, spark plug ports, cylinder head, or barrel leaks) or dynamic (ring bypass leakage)?

If the leakage is determined to be by the valve seats, verify that the cylinders were "staked" to examine the possibility that the valve seat was held open due to combustion products being lodged in the valve seat areas.

If the leakage is dynamic, or by the rings, have the engine started or the aircraft flown for a short period of time to obtain a second compression reading. In general, do not react to a single low dynamic compression reading unless there are other signs of deteriorating engine condition.

Use your dynamic compression reading as only one element of your cylinder health evaluation. Consult the TCM service programs desk for further clarification.

If the leakage is dynamic, or is at the ring to barrel seal, several factors need to be considered and some technical background is again helpful in assessing the proper course of action. Cylinder differential compression tests are conducted using 80 psi test air. During actual engine operation, cylinder pressures up to 1,000 psi force the compression rings against the cylinder wall and the actual running dynamic seal can be quite different than the static measurement. That is one reason why it is not uncommon to have a normally running engine show a low differential compression leak check at scheduled inspection or maintenance.

Other less qualitative factors also influence the dynamic leakage evaluation. The amount of oil on the cylinder wall can have an influence, with more oil leading to an improved differential compression reading. On occasion, due to the rotation of the rings in the piston, the ring end gaps can align to produce an increased leak path and a degraded differential compression test result. Piston design, cylinder choke, and other factors may affect the test result you get but not necessarily engine performance. So what is the proper course of action with respect to dynamic leakage?

TCM's recommendation is to realize that the differential compression leak test is only one indicator of cylinder health, especially for dynamic leakage. If your oil utilization pattern is not changing dramatically and the other health monitoring items described in the TopCare Health Checklist are nominal, it may be best to simply continue to monitor the situation. In the absence of other indicators of poor cylinder health or changing engine characteristics, you should not react to a single low dynamic differential compression reading.

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