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CSI: Engine Bearings: When Good Bearings Go Bad

There can be literally dozens of reasons for premature wear or failure of an engine bearing. Luckily, the criminal always leaves a trail.

By Doug Kaufman

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Engine bearings have the dual function of reducing friction between a rotating part of the engine (the crankshaft) and the stationary part (the main caps and engine block) and supporting the crank.

Because of the stresses caused by the explosions inside the internal combustion engine, the bearing material must be extremely strong, so a durable metal is required.

Reducing friction is accomplished in part by the fact that dissimilar metals slide against each other with less friction and wear than similar materials will. So an alloy bearing material does a much better job of keeping a steel crankshaft moving than a steel or cast iron bearing will.

Although the material itself may give the engine bearing some friction reducing properties, its performance is enhanced by a lubricant between the moving and stationary surfaces. Another of the bearing’s duties is to establish and maintain a film of oil.

For all these requirements, the bearings usually do a very good job at keeping the moving parts moving. However, when a bearing fails the results can be spectacular – and catastrophic.

But even when they fail, the fault usually doesn’t lie directly with the bearing itself. There is usually another, less obvious culprit behind the crime, which may force engine builders to employ some CSI-like investigatory tactics.

There can be literally dozens of reasons for premature wear or failure of an engine bearing. Luckily, the criminal always leaves a trail. By carefully inspecting the evidence left behind, an engine builder can discover and eliminate the cause of bearing failure.

Some failures will be caused by a combination of factors, but the photos that accompany this article (additional pictures can be found online at www.enginebuildermag.com) can help you determine the most likely cause of failure.

Dirt or Debris
Whether it’s dirt, dust, grinding remnants, shot or any other particle, debris can cause significant damage to a bearing surface.

If it’s in the lubrication system, dirt will usually leave circumferential scratches and often the offending piece will remain lodged in the bearing’s surface. Always be sure the lubrication system is flushed thoroughly before reassembling an engine.

Another way dirt can play havoc is if the engine components aren’t completely clean. A foreign particle trapped between the bearing back and the housing will cause a raised area in the bearing. This little hump can lead to contact between the bearing and the crank journal. You should always ensure the bearings are installed in clean surfaces.

Insufficient Lubrication
A complete lack of lubrication in the crankcase usually leads to seizure of the bearing and total failure of the engine. But experts say a more frequent issue regarding lubrication is simply inadequate oiling.

The absence of a proper oil film will result in metal-to-metal contact, occasionally to just one bearing or often to a number of them.

When the bearing is damaged by oil starvation you’ll find a very shiny surface and evidence of wiping.
Remember, the failure of the oil film can be seen in a variety of ways on the bearings. Check or such things as blocked oil passages, a malfunctioning oil pump, improper bearing selection or installation, oil seal failures, fuel dilution (often caused by blowby of fuel and air past the piston rings) or foaming or aeration, caused, ironically, by an overfilled crankshaft.

Misassembly
Sometimes, failures are the result of simple installation errors. For example, if a bearing half without an oil hole is improperly put into a position where the hole is needed, that journal will receive no lubrication.

Other types of assembly errors may also be seen. If a connecting rod or main bearing cap is installed in the wrong position, or a bearing isn’t set into place securely, lubrication will be insufficient and cause failure.

Careful installation procedures are, of course, critical in every aspect of engine building – careless errors are always expensive.