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How to check an under-run engine for rust?

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Well-Known Member
Lifetime Supporter
Jun 12, 2007
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Seattle, WA
So, I'm looking at this Champ. It's in great shape, but the engine (a Continental C90) is a giant throbbing question-mark: it's flown about 125 hours in 10 years. It was overhauled at the beginning of that 125 hours, 10 years ago. The owner keeps it in a closed hangar with a recirculating air heater, usually keeping the engine compartment at around 60 degrees F. He keeps dessicant plugs in the exhaust pipe ends. There's a 75W bulb that burns constantly in the engine compartment. The prop is never turned. Short of pickling the engine, and replacing the sparkplugs with silica gel plugs, he's doing everything right.

But. The annual for 2016 happened in mid-June, and it accumulated exactly zero (0) hours between then and when I test-flew the plane last weekend (1.1 hours). That's about 7 months of sitting perfectly still. I get the impression he looks into the hangar on occasion, but never really touches anything. This is probably not the only 7+ month period in which it's sat idle, it's just the one I'm sure of.

He says the oil has about 5 hours on it now, with the obvious implications. I don't have the logs to peruse, but it's probably about a year old, maybe more.

I, obviously, want to get some kind of reassurance that I'm not buying a nice plane and a $20k engine overhaul. It started and ran very nicely, which means little, but is still a positive sign to me. He claims 78/80 for all the cylinders. Still.

He's unwilling to have a jug pulled to take a gander at the cam (he'll just keep it, if that's my condition of purchase; he's very meticulous, and doesn't trust the local A&P to pull a jug without screwing it all up). He suggests, rightly, that doing an oil analysis with no history (he claims no history of oil analysis) doesn't tell you much, and that doing it with 5 hours on the oil tells you less. There's just an oil screen, no filter, so the only metal a prebuy is going to find will be pretty gross chunks if the engine is doing bad things.

So, this leaves me with the question: is there anything else I can do to determine the state of the engine beyond the pistons? Is that oil analysis worth it? We'll borescope the cylinders and look for obvious bad signs, but that doesn't help me if the cam secretly looks like the moon's surface. The A&P thinks replacing the cam and followers is about a $5k job, but that would make this airplane at least $5k overpriced. That also doesn't help me if there's rust on the crank or elsewhere.


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