LIQUID GASKET

HOW ENGINE OIL HELPS PISTON RING SEAL.

- LAKE SPEED JR.

No experienced engine builder would intentionally bolt a cylinder head to an engine block without installing a gasket between the block and cylinder head. The gasket seals the block deck surface to the cylinder head surface. Smooth-looking metal has microscopic surface roughness and waviness that creates leak paths, especially under high cylinder pressures. The gasket fills the voids and contains those high pressures. That's why high-horsepower engines O-ring the blocks and heads. Those same microscopic peaks and valleys exist on the cylinder walls, piston rings, and piston-ring grooves. Motor oil acts like a gasket to seal those components together.

If you've ever leakdown tested an engine-hot or cold-you've probably seen a difference in the readings. Part of that is due to the thermal expansion of the parts, but also from the amount of oil on the cylinder walls and piston rings. A cylinder with a dry wall and piston ring will leak more than one that's wet with motor oil, which is why mechanics sometimes squirt oil into a cylinder through the spark plug hole before doing a compression or leakdown check. It might seem that a smoother surface finish on the cylinder wall would be the answer, but a certain amount of roughness is needed to hold the oil, as with the block deck.

I experienced this firsthand back when NASCAR and NHRA Pro Stock teams began to experiment with mirror-smooth bore finishes. The idea was to reduce friction, but these super-smooth bores didn't retain enough oil. As a result, blow-by increased and crankcase vacuum decreased.