Risk of field overhaul
Larry,
that is an interesting question. I think with any overhaul of an engine there is likely to be somewhat more risk of engine stoppage in the first hour or so of run time. This is a risk that seems to be often accepted with any engine that is field overhauled. However while test cell running does reduce this risk I don't think it eliminates all elements of risk, especially those elements that relate to some of the largest factors leading to engine stoppage such as fuel system problems. But that doesn't really answer your question.
There are specific instructions on how much time you can run on the ground before first flight and some specific temperature limitations. I was very careful with temperature limitations but the bottom line for me was that I was not going to fly until I was absolutely sure the engine was going to be producing power for at least several minutes of flight. The risks with excessive ground running according to the eci break in manual seemed to be two fold. First was glazing of the cylinder walls which can lead to high oil consumption and excessive blow by. I didn't believe that excessive blow by or high oil consumption was likely to lead to power failure on the first flight. The second risk seemed to be overheating of the cylinder head leading to scoring of the cylinder walls which was something I could inspect for before my first flight with a small bore scope.
Ground runs were specifically limited by ECI to 4 minutes each before the first flight. I probably exceeded this a couple times as it was difficult to operate the engine at the recommended ground power settings close to my hanger. I ran the engine several times on the ground mostly so that I could assure the aux systems were correct.
I was probably less concerned about the engine core than I was about the engine aux systems, but I probably had 30 to 40 minutes of ground time before I flew, which I am sure is excessive. My friend who bought a test cell engine had 1 hour on the engine but no hours on the aux equipment such as the fuel system, prop and new electronic ignition that replaced one of his mags.
I felt that carefull assembly, ground running and inspections subsequent to ground running could reduce the risk to an acceptable level for the first flight.
All of that said, I still think the lowest risk method of engine overhaul would be to have an experienced shop perform the work and run the engine in a test cell. I'm sure that many people will argue that this practice reduces risk and is the safest option, I don't disagree with this and would encourage anyone to have their engine built by a proffesional. On the other hand I was eager to build the engine for my own enjoyment and education. I think acceptance of some risk for these benefits is the same trade off that is made when building or flying an experimental aircraft as opposed to a certified aircraft. Perhaps most people would disagee with me as most people seem to prefer to put non-amateur built powerplants in their amateur built airplanes.
I really enjoyed assembling the engine.
Good luck,
Bryan
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