Why Engines Fail: What the Bearings Tell You

A spun or wiped bearing is almost never the reason an engine died. It's the evidence. MAHLE Clevite, one of the largest engine-bearing makers, sorted premature bearing failures by cause in its "Engine Bearing Failure & Analysis Guide": dirt 45.4%, misassembly 12.8%, misalignment 12.6%, not enough oil 11.4%, overloading 8.1%, corrosion 3.7%, a poor crankshaft finish 3.2%.

Read that list again with a replacement engine in mind. Most of those causes stay with the vehicle when the old engine leaves.

When the failed engine comes out, ask the shop to pull a main or rod cap and look before the core goes back. The bearing surface usually shows which story you're in. MAHLE's guide files every failure the same way, from "Appearance" through "Corrective Action," and the appearance is the part your shop can check in five minutes.

**Dirt.** Particles pressed into the soft lining, sometimes with scratches around them. The debris came from somewhere: a torn air filter or intake leak, bits of another failing part, or an engine that wasn't cleaned properly the last time it was opened.

**Oil starvation.** The surface is polished bright, or smeared where the crankshaft wiped the lining. In the worst cases the lining is torn away and blackened by heat. Low oil level, a clogged pickup screen, a failing pump or blocked passages all end here.

**Overloading.** Small cracks and missing flakes of lining, like dried mud. Lugging the engine at low rpm under heavy load, detonation, or too much fuel will do it.

**Corrosion.** Dark, spongy, etched lining. Acids build up in oil that's run too long, or when coolant gets into it. A leaking head gasket or oil cooler is the usual suspect.

**Misassembly or misalignment.** Wear concentrated near the edges or on one side of each shell. That's a machining or assembly problem from the last rebuild, not something the next engine inherits.

This is where the money is. A new long block bolted to the same problem fails the same way, and often faster, because it's tight and fresh.

• **Oil system.** Replace the oil cooler, or flush it and its lines, if the old engine shed metal. A cooler full of debris feeds it straight to the new bearings on first start.
• **Air intake.** Check the air filter seal, the intake tube and every coupler for a path that lets dirt around the filter.
• **Coolant.** If the old oil was milky, find the leak. Pressure-test the cooling system and inspect the oil cooler before the replacement is filled.
• **How the vehicle is used.** Towing near the rating in too high a gear, or a tune that adds fuel, will overload the new bearings exactly like the old ones.
• **Oil.** Use the grade and quantity your manufacturer specifies, and prime the system before the first start so the bearings never turn dry.

Unlike a transmission, whose risky stretch is the first 500 miles, an engine's riskiest moment is the first minute. Oil pressure has to come up right away.

A used engine's bearings are the one thing you can't see without opening it. The seller rarely knows why the donor vehicle was retired either. A remanufactured engine is torn down and rebuilt with new bearings on a crankshaft that's been measured and reground or replaced, so the dirt and wear from its previous life don't come with it. [Our comparison of remanufactured and used engines](/blog/reman-vs-used-engines-transmissions-how-to-choose) walks through what each option gets you.

One thing no teardown can tell you: whether the next driver lugs it up the same grade with the same trailer.

• Overheating after engine or transmission replacement: /blog/overheating-after-engine-or-transmission-replacement
• Used and reman engines: /used-engines