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We've been having some discussions around if there was a specific ruling from the FAA that explicitly said that if a cylinder was showing less than 60 on the compression test, the FAA deemed it unairworthy. I've heard that, it's a rule-of-thumb and not a ruling, but I don't know for sure.

Has this been conclusively made into a ruling from the FAA, or does it exist as a rule-of-thumb and there are other factors to consider?

This would be specific to United States/FAA and 4-6 cylinder Lycoming or Continental piston engines.

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Long story short is that the manufacturers instructions take precedence over all. FAR Part 43 Appendix D requires that at each annual and 100 hr the compression be checked and that if there is "weak" compression further inspection of condition and tolerances of the engine have to be made. Now the term "weak" is kind of broad. FAA Advisory Circular AC43.13-1B states:

If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine, and procedures in paragraphs 8-14b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected.

However, it also states:

This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions.

So the gist of it seems to be that if the compression is below 60/80 on a hot engine IN GENERAL, then yes it needs to be removed and inspected, UNLESS the manufacturers instructions state otherwise. Keep in mind Advisory Circulars are guidelines and not regulatory in nature although they are usually used as best practice guides. There is a great AVweb article The Savvy Aviator #13: Putting Compression In Context that has more in depth analysis.

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    $\begingroup$ Mike Busch also did a webinar for EAA called "All About Cylinders" which talks about compression tests (among a whole bunch of other things) - it's worth watching. $\endgroup$ – voretaq7 Jan 13 '14 at 17:16
  • $\begingroup$ Keep in mind that for Part 91 operations manufacturer instructions and service bulletins are guidelines. Compressions are interesting data points but do not give the whole story about cylinder health. A borescope inspection is the best way to determine whether a cylinder needs to be replaced. I currently have one cylinder on my C210 that is below 60/80 and a borescope shows some leakage at the exhaust port, but otherwise the cylinder is fine. $\endgroup$ – JScarry Mar 27 '17 at 16:39

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