MMO, Silica Plugs and Pickling!

Recent flight to Rutland

I dug the old C75 Continental, the airplane engine for my L2-M, out of the shop recently. Ten years ago, when I had decided to stop flying the L2-M and rebuild it, I bolted the engine to a stand, covered it with a tarp, and shoved it in a corner. The engine itself is fairly small and looks like it came out of a Volkswagon Bug. It is an air-cooled 4 cylinder job that puts out 75 horspower. I needed to transport it from my dusty farm shop to the SD Airservice Hanger in Rutland, where we could start getting it ready for installation in the old warbird.

Rust, and corrosion are one of the dangers of having an engine sitting around and not being used for long periods of time. There are procedures you can do to “preserve” an  engine. One is called “pickling”. With this method, you drain all oil from the crankcase and oil pan and fill it with a special preservative oil, install silica plugs (more on them in a minute) and wrap the whole thing in airtight plastic wrap. The whole point is to keep moisture out of the engine, where any exposed metal parts, like the inside of the cylinder walls, could rust. This is the way it should be done…not the way I did it.

Old MMO add

First of all, I did not expect this rebuild project to take 10 years! As my engine sat in the shop corner, I had left the propeller attached, and had installed the silica plugs. I squirted  Marvel Mystery Oil (MMO) into the cylinders and would “swing the prop” every month or so to keep everything lubricated. This was my “farmer’s approach” to engine pickling.

With the engine safely in Rutland, on a new heavy-duty engine stand welded together by my friend and neighbor Nate Smith, I went about cleaning and inspecting. Things looked pretty good, considering, but I did need to refresh those silica plugs.

Silica plugs are made of glass or plastic and are designed to screw into the spark plug holes on the top of the cylinder and collect any moisture trying to get in. These glass tubes are filled with silica crystals and are a bright blue in color. When they absorb moisture, they turn pink; a sign they need to be baked or replaced. I baked them.

Cylinder with a silica plug installed

Using the local Rutland Chapter of the E.A.A. meeting room/kitchen, I pre-heated the oven to 400f, and made a tin-foil tray. I disassembled the four plugs, emptied the silica crystals onto the tray, and baked until they turned bright blue, indicating they were moisture free. Then, very carefully, holding various tiny plug parts together, I poured the hot silica into the plug tubes. The plugs are then ready to stick back in the engine!

Silica plug with baked and dry crystals

Crystals are dry

Crystals are dry and ready to pour back into plugs

silica plugs

Left silica plug shows pink crystals and the right one are blue, or dry, crystals

Work on the L2-M in Rutland will be a bit sporadic while sugaring season gets into full swing at the farm. Look for future flying stories about the “Master Orifice Tool”, and the “Damn Dashboard Mice”. There will be some more sugaring stories from the farm as well.


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