Playing Hide and Seek with Oil Pressure

Over the last two weeks, my Bede BD-4C airplane has been in the shop for focused work on the engine. I got a couple of small tasks done while it was there but progress has been largely stalled. I asked the A&P mechanic to make some final adjustments to the engine so that it would be ready to fly:

• Set the timing of the magnetos. Specifically, set the right mag (since I had removed it so that I could tighten a nut that I could not reach with the mag in place) and quickly double-check the timing of the left mag.
• Set the idle speed.
• Set the mixture.
• Advise me of anything else that he saw which needed attention before first flight.

This should have taken a few hours, tops.

He set the timing of the magnetos. He found a few things which needed attention, mostly lack of safety wire on a couple of items. We motored the engine over on the started. No oil pressure.

He checked all sorts of things. We tried again. No oil pressure.

He continued to check things. And try workarounds. We continued to try stuff. No matter what, we never found any oil pressure.

After a week, we agreed that the best course of action was to remove all of the accessories from the back of the engine (magnetos, backup alternator, etc.). Then we would remove the accessory case. Then we would be able to inspect the oil pump.

None of this should have been necessary because the oil pump on a Lycoming engine is stone simple. Here is the whole thing. (Click to see the diagram larger.)

The oil pump shaft (8) slots into the end of the crankshaft. If the engine turns, the oil pump has to turn also. There is no way for it to not operate. Here is what the back of the engine looks like with the accessory case removed. You can see the back end of the crankshaft, with the slot for driving the oil pump, between the two larger gears in the middle of the photo.

Here is the accessory case. You can see the end of the oil pump shaft, which fits into the slot on the crankshaft, at the top of the “V” of safety wire.

The oil pump, it turned out, was just fine.

Further inspection with a flashlight and mirror did find the problem… fortunately. This plug had been installed, blocking the path from the oil sump to the screen and thence to the oil pump.

The engine had previously been installed on an aerobatic airplane and had had a Christen inverted oil system. The external components of that system had been removed but, unbeknownst to us, the 805 sump plug had not been removed. You can see it in this diagram from Christen’s documentation. Look at the Horizontal-Screen Sump at the bottom of the image.

With the plug out and the accessory case reinstalled, turning the propeller through by hand made oil circulate properly. We fully expect that the engine will develop oil pressure when he finishes reassembling it next week.

The bummer is that this has taken so much time that I will not be able to fly my Bede BD-4C airplane to Oshkosh for AirVenture this summer. It will fly, though, and I would much rather do this safely and at a reasonable pace than rush.