The Cessna 120-140 Association Forum

Last week I noticed a couple momentary spikes in oil pressure! I was a bit baffled to figure out what would cause that. Obviously something had to be blocking flow downstream of the pump. I had 47 hrs on the new filter so started an oil change when I landed.

A few small bits of debris came out the quick drain and were caught in a paint strainer! NOT GOOD!

I opened up the filter and....METAL! my engine has 1808 SMOH. I immediately enter an emotional spiral dive contemplating a teardown and overhaul over the winter.

I collected all the debris from the filter, determine there are at least a couple types of material in there, some ferrous, some not. It's chunks, not shavings. Feels like gravel. Lots of hypothesizing with friends about where it could be coming from.

Planned next step is to drop oil tank and see if there are any clues. Followed by the generator which I felt had been wetter than "normal" later and leaking oil.

In the bottom of the tank...BIG PIECES. This is good! Identifiable parts! No part #s readable, haha, but it didn't take long to identify that it was the drive coupling from the generator that had self destructed. I'm also relieved that the main parts of the engine aren't coming apart

I then discover there is a Service Bulletin to check the coupling every 500 hrs. For good reason! Add it to your maintenance program!

I'm glad that i caught the spikes on the engine monitor. It's easy to want these indications to be an instrumentation error etc. But your engine is talking to you, so listen!

Ill probably go ahead and convert to an alternator now...

Engine monitor graph showing spikes.

Holy cow. The fragmented bushings and mangled retainer all made their way down into the oil tank?

I just attempted SB95-3B and ran into some interesting challenges. Just a heads up that drive gear 531325 ($$$) seems to come in two flavors with different inner diameters. That part number should fit around sleeve 530407. Mine contained a drive with a smaller inner diameter to 531325 and no sleeve. I cannot explain the variability compared the service bulletin.

Here's a copy of the Service Bulletin for future reference.

If you have the Ford alternator, you can stick your finger in the opening where the cooling fan is and turn the shaft of the alternator by pushing on the fan. (WHEN THE ENGINE IS NOT RUNNING!) There will be a surprising amount of "play", maybe half an inch or so, but if your coupling has failed it will be unmistakable. I check it at every pre-flight.