I ran across this recently and thought it was worth posting. It's not new info for many people, especially those who build a lot of engines. However, there are fairly regular questions posted about "What oil weight should I use?", and I'm hoping the info below on viscosity vs. bearing clearances can help people figure this out more logically.

Also, any "real world" experiences that people can post here which seem to fall outside of these guidelines (successes OR failures) would be of interest, too.

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Years ago, NASCAR engine builders discovered that if they ran crankshafts with smaller diameter journals, they could pickup some extra horsepower.

Reducing the circumference of the bearings reduced friction. A crankshaft with smaller journals was also lighter and revved faster.

The smaller journals also reduced the strength of the crank, so there was a limit as to how far they could go without dangerously weakening the crank.

Also, reducing the diameter of the journals increased the loading on the bearings. The larger the bearing, the broader the area over which the load is distributed.

The smaller the bearing, the more concentrated the load is over a given portion of the bearing. Tightening up the bearing clearances and using lower viscosity oil helps distribute the load over a broader area of the bearing.

Regardless what type of bearings are used in an engine, clearances have to be precise and correct for the oil viscosity that will be used to lubricate the motor: Thin, low-viscosity motor oils require tighter bearing clearances than heavier-viscosity racing oils such as 20W-50, straight 40 or 50 weight oils.

Some recommended rod bearing oil clearances for various oil viscosities:

.0015 to .0018 for 0W20 or 5W-20

.002 to .0024 for 5W-30 or 10W30

.0025 to .0029 for 10W-40 or 15W40

.0030 to .004 for 15W-50 or 20W-50


For main bearings:

.0015 to .002 for 0W20 or 5W20

.002 to .0025 for 5W-30 or 10W30

.0026 to .003 for 10W-40 or 15W40

.0031 to .0041 for 15W-50 or 20W-50

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