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Q-talk 105 - Oil Leak Symposium (From Q-List)

The best solutions I have found for oil leaks are these: Crankcase halves should be reasonably flat, mating surfaces. They should be cleaned and dressed with a light faying surface of Hylomar. This material is a light blue translucent and does not fully harden. It is designed to just fill in the surface imperfections between metal-to metal joints, not to create a gasket having any thickness. The halves are joined and torqued to the specified value. Joints having a "gasket" thickness of sealant tend to lack structural integrity and have a greater opportunity to lose torque, shift, fret, and leak.

Hylomar is very nice also because it is extremely easy to remove upon disassembly. You can find it at Kragan Auto Parts packaged by Permatex. The stuff is made by Loctite. Aluminum crankcases are often plagued with porosity. They can be treated during teardown by rod peening or shot peening (not bead blasting) the inner surface. They can also be treated by a process called "impregnation" that involves a pressure treating process. Carroll Shelby's wheel shop used to pressure test wheels and when a leak was detected, they would use a rod penning gun to bend the metal closed from the inside of the wheel. A casting shop, where I once worked, used the impregnation process on all A3 56 permanent mold aluminum castings. Some version of these processes should be aircraft certified and available at your local repair station. Crankcases most often leak when there happens to be too much internal crank-case pressure being generated by combustion chamber blow-by. Either rings or valve guides can contribute to this. I know that aircraft engines are designed to have lar-ger-than-auto clearances, but even on my 1948 Vincent Black Shadow (something with huge air cooled cylinders from post war Britain), a simple test could be performed with a balloon on the crankcase breather. The crankcase breather should not consistently produce so much pressure that the balloon would be inflated. While this principle may not apply on an aircraft engine, I make the point that if the crankcase is not breathing properly, there is a good chance that oil will be forced by excessive pressure to leak oil. The cylinder and valve condition have a great affect on how much a crankcase leaks.

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