In order for bearings to maintain a stable oil film, which is vital to their life, there must be a perfect match with clearance size, oil viscosity, engine operating temp, crank surface finish and geometrical alignments. General rule of thumb for determining oil clearance is .001 of clearance for every inch of shaft dia. Ex .001 x 2.00 shaft dia. = .002 oil clearance. For clearances smaller and larger than the rule of thumb ,refer to the factors listed below:
Factors for tighter oil clearances:
- Smaller journals
- Lower viscosity oils
- Surface finishes min 4 RA
- Near perfect crank and engine block geometry
- Higher engine operating temperature due to decreased oil flow
- Perfectly balanced cranks
Factors for looser oil clearances:
- Larger journals
- Higher viscosity oils
- Surface finishes rougher than 5 RA
- Not so perfect crank and engine block geometry
- Lower engine operating temp due to increased oil flow
- Weaker housing bores causing distortion at high RPM’s
Engine bearings should operate with the least amount of oil clearance as possible. Tighter oil clearances produce less peak loading on the bearings and the engine runs smoother with less vibration. Remember, you can mix bearing sizes offered by King to achieve desired oil clearances. Ex. Pair a STD half bearing with either a .001 or STDX half bearing to obtain your clearance.
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