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Triumph TR3 FAQ page

Bearing Lubrication

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

 

 

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