The folks at Three Wing Flying Services got started on N631S's annual inspection yesterday, a day late. Mike the IA did the engine runs, uncowled the airplane, dropped the old oil and checked the cylinder compressions as instructed by TCM Service Bulletin SB03-3.
One cylinder, #2, was not very tight. It tested at 48/80 psi on a day when the minimum acceptable value based on the standard orifice was 46/80. I've gone to the logs and dug out the compression values at each annual inspection since 2007 (charted below).
You can see that #2 has always been the "weak sister" among the cylinders. Since the result has to be considered in relation to the minimum acceptance value and that in turn varies with atmospheric conditions, I made another chart (below) that shows the difference between the test result for each cylinder and the applicable minimum value.
It's also clear that #4 experienced a considerable drop this year. But that's just one data point so we'll simply note the result and see what next year's annual brings.
Over the four years since the 2007 annual, N631S's engine has accumulated a bit over 500 hours. It's currently at about 1,340 hours since overhaul. That's really pretty good for a Continental engine, as they are noted for needing a "mid-life top overhaul".
Mike will inspect the cylinders using a borescope, and assuming he doesn't see anything unusual no action is needed. But I'll be watching for any evidence of further deterioration of the #2 cylinder. The engine still does not consume any significant amount of oil between changes so if oil consumption goes up the #2 cylinder will be the likely suspect.