Just a clarification about bearing lubrication. The actual oil film that keeps
the crankshaft and camshaft from touching the main, rod or cam bearings is not
the oil pressure generated by the oil pump. The oil pump supplies oil to keep
the space between the crankshaft and the bearings full. As the crankshaft
rotates, it does not remain in the center of the bearing clearance space, but
it is constantly being pushed around in that space as it rotates by the loads
that are applied to it. As it moves around, it pushes a "wedge" of oil just in
front of the spot where the maximum load is applied at that moment. The
crankshaft "rides up" as it were, on that wedge oil. That wedge of oil is
what keeps the crank separated from the bearings. If the supply of oil is
interrupted, for even and instant, the wedge or oil is lost. Then all that is
separating the metal parts is the "film strength" of the oil in question.. The
film strength is determined by the type of base stock of the oil, and the
additive package. The viscosity of the oil plays a part in this to a certain
extent also. Thicker oil is harder to force out from between the moving parts
than thinner oil is, but the viscosity is probably more related to the
clearances between the parts. With tighter clearances, you can run thinner
oil, but tighter clearances require more rigid and stiffer parts that will not
flex or bend. After market steel rods are not only much stronger than the
originals, but are much stiffer, and hold a rounder hole for the rod bearings.
But due to the 3 main bearing crank, the lack of journal overlap & the stroke
of the engine, the crank is not very rigid, no mater who made it. Under the
loads of high horsepower & high revs, it moves and bends all over the place.
A friend who used to work on AA Fuel drag Chrysler Hemi engine told me
they had terrible problems wiping out the main bearings because of crank and
block flex until they gave the main bearings .010" or so clearance and used
70 SAE weight oil to keep the pressure up. This let the crank and block move
around without actually touching one another.
Greg Solow
The Engine Room
Morgan & Sports Car Specialists
Santa Cruz, Calif. 95060
831 429-1800
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