How fast does main bearing wear happen?

[Gareth Randall]

I have a 21-year-old Jeep Grand Cherokee 4.0L. It’s running great with oil pressure well within factory spec at both idle and above 1600rpm, but I was wondering – can I make any kind of prediction about how long it will take before the bearings wear so much that the pressure will be too low?

Just FWIW, factory spec oil pressure for the 1993 4.0L is minimum 13psi at idle and 37psi above 1600rpm. I’m getting about 40psi cold idle/30psi warm, and 50psi above 1600rpm.

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[Pat61]

Not really, but you could have a lab do an oil analysis annually. And speaking of bearings, my buddy just rebuilt a ’58 vintage IHC 6 cylinder truck engine. Some of the parts were unobtainable and, you guessed it, the main bearings got reused. They appeared to be in good shape, had no dirt imbedded in them, and had survived excessive over-speed even tho the old engine had tossed a rod. Plastigauge confirmed our thoughts and the rebuild continued.
Do the maintenance religiously and the bearings will be the last thing you’ll have to worry about.

[Bill]

That engine uses 7 main bearings and are not usually a problem. Rod bearings wear out in a 4.0lt. before the mains do. I have seen them wear out pistons and cylinder walls and the main bearings are still perfect.

[Author]

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