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Bearing clearance
The right bearing clearance is crucial for the oil film and durability. Enter the journal diameter, block material and oil grade, and you get a guiding cold clearance. Always measure the actual clearance during assembly.
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Recommended cold clearance
0,054 mm
Guiding cold clearance. Always follow the maker's specification and measure the actual clearance (plastigage or dial gauge).
How the calculation works
The rule of thumb is roughly one thousandth of an inch of clearance per inch of journal diameter for a cast-iron block. Aluminium grows more when hot and therefore gets slightly less cold clearance. We also adjust for the oil viscosity, since thicker oil tolerates and wants a little more clearance.
This is a guiding starting point, not a definitive answer. The real clearance depends on the engine type, the use and the maker's specification. During crankshaft reconditioning we measure the journals and bearings and document the clearance so it lands within the right tolerance for your particular build.
Example
A 54 mm journal in a cast-iron block with 10W-40 gives a guiding cold clearance of around 0,054 mm (about 0,002 inch).
Factors for bearing clearance
| Factor | Guide value |
|---|---|
| Cast-iron block | approx 0,001 inch per inch of diameter |
| Aluminium block | approx 0,0008 inch per inch (grows more when hot) |
| Thin oil (0W-20) | less clearance |
| Thick oil (20W-50) | more clearance |
Guiding. Always follow the maker's specification and measure with plastigage or a dial gauge.
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