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Warning: my gearbox acted up during a routine runup on the ramp in Houston last Tuesday
I was doing a runup on a Cessna 172 after an oil change, and the right mag dropped way more than usual. Figured it was just fouled plugs like always, but after I pulled the cowling I found the gearbox had a hairline crack near the oil return line. Scared me enough that I swapped my whole inspection routine to checking those areas every 50 hours now. Has anyone else seen gearbox cracks on older Lycomings?
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nancym345mo ago
Check the oil return line bracket attachment point while you're at it. On older Lycomings, I've seen that bracket itself develop fatigue cracks where it meets the accessory case, not just the gearbox casting. A friend had one fail completely in flight on a 172 out of KSEE - the bracket let go, the line started whipping around, and it dumped enough oil to drop pressure before he got it shut down. I clean the area with brake cleaner, then use a 10x loupe to inspect both the gearbox casting and that bracket edge every annual now.
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wood.zara4mo ago
Notice how it's always the small, overlooked spots that cause the big failures, just like @nancym34 pointed out with that bracket...
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murray.cole5mo ago
That "hairline crack near the oil return line" is exactly the spot I've been paranoid about on my O-320. I started looking closer at that area after a buddy of mine found a crack on his engine during a prebuy inspection. It was so small you could barely see it with the naked eye, but once he cleaned the area with solvent and hit it with a bright flashlight, it was a clear stress fracture. I think the vibration and heat cycles over time just make that casting weak right where the oil return line puts pressure on it. Now I always run my hand along that whole section with a rag during every oil change, feeling for any rough spots or lipping that could hint at a crack starting. It's a solid call to add that to your 50 hour checks, because catching something like that early could save you a whole lot of trouble and money down the road.
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