Bellanca Engine Overhaul:
Time for an Overhaul
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In late 2000 it became obvious that my Bellanca was in need of an engine overhaul. The engine type is a Continental O-470-K horizontally opposed "flat" six cylinder air-cooled aircraft engine. The engine was running ok, but the compression, which is a good indicator of the health of an engine, was sagging into the 50's (a perfect score is 80, and 60 is generally considered the lowest allowable value). The engine had about 1200 hours total time, which was getting to be "high time" since the recommended time between overhaul (TBO) for an O-470-K is 1500 hours.
In many cases this would be a situation where a top overhaul would be appropriate. A top overhaul is where you remove and overhaul the "top end" of the engine: cylinders, pistons, and valves. During a top you do not disassemble or overhaul the "bottom end" of the engine which consists of the connecting rods, crankshaft, camshaft, engine case, and accessory gear train and drive pads.
A top overhaul would be appropriate where the condition of the bottom end of the engine is known to be good, and you want to squeeze out some more operation before going in for a full overhaul at the recommended TBO interval. Tops are relatively cheap and don't take as much time. Tops are often performed on big-bore Continental engines (of which the O-470 is one) at mid-life, so it wouldn't be unheard of. Getting 300 or so more flying hours, which is 3-5 years of flying for me, for a few thousand dollars is a reasonable investment.
I decided to go with a full, or "major" overhaul, because of the calendar age of the engine. The airplane was built in 1958 and in late 2000 had relatively low time for the calendar age of the airframe (1200 hours). The logbooks were spotty, so it was impossible to tell for sure, but it was highly likely that the engine had never been disassembled since it left the factory. This assumption was borne out by older hardware and many leaking oil seals where the rubber had hardened.
Given the calendar age of the engine, and given a long interval that the engine sat unused during the 1970s, I decided the best course was to do a major overhaul instead of a top. A top would probably have fixed the sagging compression, but it wouldn't have addressed any internal issues such as camshaft corrosion, and further it wouldn't have addressed age issues such as the cylinder casting styles being long out of date and hard-to-fix oil leaks due to old and cracked seals.
Since I'm something of a mechanical perfectionist, it wasn't that difficult to take the plunge into a full overhaul. Once decided on the course, I then needed to decide who was going to overhaul the engine.
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