Oil Pump & Accessory Housing

Things haven’t been going so well for my little airplane these days. I keep getting stranded away from home because my oil pump won’t seem to stay primed. When I start the engine, it fails to make oil pressure. There are all kinds of hocus pocus tricks you can try to get the pump primed again and I’ve tried them all. TWICE!! I finally reinstalled my custom made fitting on the left side of my engine case and used a T-fitting to add a primer line (the orange line in this picture). It worked, but not for long…

The long and the short of the situation is that my oil pump gears and/or the oil pump housing had worn beyond tolerances and could no longer make pressure. This is my accessory housing after I removed it from my engine. The plate in the middle covers the oil pump itself, which consists of 2 hardened steel gears.

Now: I’m no engineer, but even I can see where putting 2 hardened steel gears inside an aluminum housing might lead to some wear issues.

This is the aluminum plate that covers the oil pump and holds the gears in place. You can see where the gears have caused some wear marks on the plate. A lot of people are successful in restoring oil pressure by lapping this plate and reinstalling it. By the time I had gone to the trouble of removing my accessory case, I was going for the sure thing… not a maybe.

Here are the gears in the housing. The square drive on the lower gear engages on either the crank or the cam (can’t remember now) which then spins the gears. The backside of the top gear (not shown) has the square drive output for the tachometer.

You can see the wear on the housing from where the hardened steel gear has rubbed. There was a similar wear pattern on the other side of the housing, too.

I could have the old housing remanufactured for $850 or purchase a new housing for $905. The new housing is on the right. :-) Ouch.

The new accessory housing from the backside. Sure is pretty with that gold finish…

I’ve never actually painted a chess set, but I imagine it to be very similar to taping off and painting this accessory housing.

I reinstalled the accessory housing. Sounds simple enough, right? Wrong. I needed all new gaskets for everything attached to the accessory housing including: Both magnetos, the starter, the generator (capped), the tach drive housing and, yes, even the oil sump. By the time I purchased the accessory housing, a new oil pump kit (gears and the aluminum door) and all the gaskets, I had spent the better part of $1500. All I can say is that I’m glad I could do the work on my own because I’m sure it would have been another $1000 in labor if I was paying somebody else to do it!


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