Differential Testing of Cylinders

[Area 51%]

Because the cylinder runs hotter at the top, the taper (as measured when the cylinder is cold) accounts for the extra expansion in that area when the engine is at operating temperature.

[Onex107]

Because the cylinder runs hotter at the top, the taper (as measured when the cylinder is cold) accounts for the extra expansion in that area when the engine is at operating temperature.

Yes, that's why they are tapered. But you don't test compression when they are that hot. So there is no need to know the compression below top dead center. That test has served certified engines for many years. You can hear a bad exhaust valve in the tail pipe, and a bad intake, which is not likely, in the carb, and bad rings in the oil filer tube. Now there are optical methods to look at the valves but it's still a matter of conjecture as to how bad they are. I catch my crankcase blow in a filter jar and measure the volume of oil that is blown out of the case. It was usual to use a quart in 10 hours on the Continental. The Aerovee is passing about 10 oz. in 25 hours. All three of these methods, and the condition of the spark plugs, give you a pretty good indication of the condition of your engine. Don't wait until you see metal in the filter.

[GraemeSmith]

Now there are optical methods to look at the valves but it's still a matter of conjecture as to how bad they are.

Valve condition interpretation is covered by this AOPA ASI poster.

14FN0000-ASI-Valve-Safey-Poster_Final.pdf

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[Area 51%]

Yes, that's why they are tapered. But you don't test compression when they are that hot. So there is no need to know the compression below top dead center.

Since a "cold" engine IS narrower at the top than farther down the barrel, that's exactly where a compression test shouldn't be done. Further down the pipe would reveal a more realistic indication of the overall condition of the cylinder. Unfortunately, since the valves are in constant motion everywhere except TDC, that's the only place a static compression test CAN be done. Unless you are willing to remove the rocker arms.

[Sonerai13]

Robbie,

Others have beat me to the punch, but looks like all the posted answers are correct. 14 CFR 43 appendix D is referenced by your operating limitations, so it is your guide for the condition inspection. Within 43 D there is the requirement to test the compression of the cylinders. Method is up to you.