Bearing Clearance and Oil Viscosity Explained
The relationship between bearing clearance and oil viscosity is a delicate balance. Read on for more information on how to get it right!There are some fairly famous duos in the entertainment industry – Abbott and Costello, Rogers and Hammerstein, and Batman and Robin among the more illustrious. So it may not be much of a stretch of the imagination for an engine guy to include the pairing of bearing clearances with oil viscosity as a duo you should become acquainted.

The typical rule that most engine builders follow is that bearing clearance establishes the viscosity of the oil to use within a given oil temperature range. Generally, the tighter the bearing clearance, the lower the viscosity the engine can use while wider clearances require thicker oil. But there are plenty of variables that come into play that are worth investigating.

Viscosity is a rating system for oil that rates the oil’s thickness at a given temperature. Production engines are increasingly moving to thinner, less viscous oil in search of improve fuel economy but there are performance advantages to thinner oil if the bearing clearances are somewhat tighter.

The traditional bearing clearance standard for street and most performance applications has been 0.001-inch of clearance for every one inch of crank journal diameter. So a typical small-block Chevy 2.200-inch rod journal would call for a bearing clearance of 0.0022-inch. Some add another 0.0005-inch to be safe – making that 0.0027-inch.This standard works very well especially for engines using production tolerance parts where there could be minor issues with out-of-round or tapered journals or even minor errors in measurement.

This approach isn’t quite as colloquial as it may seem. There are several contributing factors that directly affect a given bearing clearance. The top three variables are load carrying capacity, the amount of oil flow allowed by the clearance, and localized oil temperature. These three are best considered together as opposed to individually since each affects the others in important ways.

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