Sunday, April 29, 2012

Preventing Oil Leaks

The only reason we use gaskets is because we can't machine a truly flat surface. Bugatti engine blocks were hand scraped to ensure that the surfaces were so flat that gaskets were not required for sealing! Almost true - they were hand scraped but Bugatti engines did leak oil.

"... if they were perfectly flat and parallel to each other and stayed that way in operation, then no gasket would be necessary." Gaskets and gasketed joints - Page 90

I suggest that both of these statements do not tell the whole story when it comes to oil leaks at joints (gasket or no gasket).

Molecules and atoms travel across boundaries if the molecules or atoms are smaller than the voids present in the material. Two examples: a balloon filled with air soon deflates as the air molecules diffuse through the rubber, atomic hydrogen atoms diffuse into steel at a rate said to be similar to salt dissolving in water. This is the cause of hydrogen embrittlement in high strength steel. The key then to preventing leakage is by reducing the size of the voids to something smaller than the molecules we are trying to contain. Fortunately, oil molecules are much, much bigger than hydrogen atoms but they are much, much smaller than the tool marks on our faying surfaces.

No two surfaces are perfectly flat and parallel but even if they did exist there could still be voids that cause leakage. If the surfaces are perfectly flat and parallel, then how do voids occur? Through surface contamination. Surface contamination creates voids by two mechanisms:

  1. Dirt and debris that prevent surfaces from clamping together. A classic example in our shop was a hair that we found lodged within the joint that prevented full seating on a non-gasketed joint. 
  2. Fluids (such as engine oil) coating the surfaces when the joint was assembled.

The first reason is self-explanatory as any particles prevent the surfaces from coming together. The second reason requires further explanation. Even perfectly flat and parallel surfaces wet with oil but fully clamped together are separated by a fluid film that allows oil molecules to travel through the joint.

Ordinary machined surfaces contain cutting troughs from the cutting or grinding tool that fills with oil, No matter how tight you make the joint the pliable gasket material cannot fill the void occupied by the incompressible oil. Voids occur where the gasket is pressed against the fluid film rather than the joint surface.

Oil travels across the joint by wicking or capillary action. Ordinary this is desirable trait of oil as oil will return when rubbed off a surface (example, ball bearing contact point where the oil is squished out of the way but quickly returns). In joints however, the movement of oil by capillary action presents a slow weep of oil.  The fix is obvious, surfaces need to be clean, dry, and free of any oil. I suggest cleaning joint surfaces with  a solvent such as MEK or isopropyl alcohol appled to a soft rag or even cotton ball. For example, when replacing a cylinder on an engine, the crankcase mating deck is often wet with oil. Clean and dry both the crankcase deck and the cylinder mounting flange.

Also, don't forget to inspect the surfaces for flatness and parallelism. See Also: Oil Leaks -- It's not the gasket, it's the surface

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