General guidance for people who work on engines. For exact figures always follow the vehicle or engine maker's workshop manual.
A failed bearing is usually a symptom
Replace a worn or damaged bearing without finding out why it failed and the new one often fails the same way. Look at the old shells, the shaft and the oil system first; the pattern of wear usually points to the cause.
The usual causes
- Lack of oil. A low oil level, a weak pump, a blocked passage or a worn engine that cannot hold pressure. The surface overheats, smears and can seize.
- Dirt and particles. Dust past a poor filter, metal debris or cleaning residue left in the engine. A bearing can embed small particles, but only so many; hard ones score the shaft.
- Misalignment and distortion. A bent connecting rod, a tapered or out-of-round journal or a distorted housing loads one edge of the bearing, which wears or cracks there.
- Overload. Knocking, labouring at low revs under heavy load or an over-tuned engine push the load past what the lining can carry and cause fatigue cracks.
- Corrosion and tired oil. Oil kept in use too long, or oil diluted by fuel or coolant, can attack the surface of the bearing.
- Wrong fitting. The wrong size, a shell that is not fully seated, a locating tab out of its notch, a dirty back, or bolts not tightened to specification.
How to prevent it
- Use the oil grade and change interval the manufacturer specifies, with a good filter.
- Before fitting, clean everything and inspect and measure the journals and housing bores.
- Keep the backs of the shells clean and dry; put assembly lubricant on the running surface only.
- Tighten to the specified torque, in sequence, and replace any bolts the manual says must be replaced.
- Turn the engine over by hand before the first start, and make sure oil pressure rises at once.