Single Bearing Seizures

Localized crankshaft deflection causing galling at one bearing/journal; oil flow discontinuity; why during build weren't measurements recorded for running fit?

Often, these problems can be caused by these, or combinations of these situations; keep in mind these are just some of the potential problems/situations:

Failure to properly control dimensional tolerances.

Problems with oil supply.

Localized deflection of crankshaft, imparting high loads (often on center bearings)

Improper bearing/journal design (insufficient diameters or widths of bearings)

Improper material selection.

Longitudinal movement of crankshaft.

Defective machining practices.

Initial design flaws (example, incorrect journal radius).

Improperly balanced rotating assemblies.

Assembly deficiencies.

Excessive/problematic combustion loading (pre-ignition/detonation).

Localized "hot spots" (insufficient cooling)

Again, just a few of the potential problems, or combination of problems.

Single rod bearing failures in a multi-cylinder engine almost always result from one or both of the following:

1. Insufficient oil flow to carry the heat out of the bearing resulting in temperature buildup which lowers oil viscosity which in turn decreases the load carrying ability of the oil film and it's thickness which in turn allows metal to metal contact.

2. Too little clearance in all or a part of the bearing to begin with which in turn leads to the progression described in 1. above.

It's important to note here that the measured clearance in a bearing is the bearing i.d. after it is installed in the rod minus the crankpin diameter. Typically in automotive engines around .02 to .03 mm. Under zero load the oil film thickness is half of that and uniform around the diameter with the crankpin centered in the rod bearing bore. As the load increases the crankpin is pushed toward the bearing surface. But if the whole thing is rotating at the same time the viscous oil is being dragged into that thin space where the load is concentrated. At engine speeds the dragging of the oil creates enormous pressures on the order of thousands of psi, enough to keep the surfaces from touching if everything is working right. That is what we're talking about when we speak of film thickness. The film thickness or actual separation of crankpin and rod bearing might be only .003 mm; but the surfaces do not touch if it's all working right.

The clearance problem in a new engine is usually the result of poor machining resulting in misalignments that may not be caught in the mandatory clearance check during assembly of any new parts or remachined surfaces or simply not doing the clearance check at all and catching an assembly mistake like excessively close bearing clearances.

Excessive loads on bearings beyond what they were designed for can produce insufficient oil film thickness and load carrying capability allowing metal contact or just excess heating of the oil in the bearing. An example is horsepower increasing modifications (like adding a turbocharger) and to a lesser extent uncontrolled detonation.

Anything that thins the oil like higher crankcase oil temperature or fuel contamination reduces the margin the bearings have to prevent metal to metal contact. And it is reasonable to expect the worst fitted bearing to be the first to fail.

Another consideration of why only one bearing usually seizes is that the failure is so sudden and immediately reduces the loads on the other bearings that even if they are close to failing they do not show any such evidence. The reason for this is that the failure is a "runaway" heat buildup and closing of the clearances due to thermal expansion.

In post #2 Standarded points out a lot of the engine characteristics that lead to the failure condition in the bearings I have mentioned. One he didn't mention is what happens sometimes when a single worn bearing is replaced with a new bearing in an older engine. If the output of the engine oil pump is marginal with respect to it's ability to maintain oil pressure in an engine with worn and excessive bearing clearances the oil flow to the old worn bearings will be favored and the new bearing with tight clearances will end up being starved for oil flow thus allowing it to build up destructive heat more easily. Since much of the work of installing bearings involves the removal of the oil pan it is usually better to replace all the rod bearings rather than just one. In an emergency situation where there are only resources to deal with one bearing then the mechanic should try to set its clearance equal to that of the other bearings.

Ed Weldon

You have received some very good answers. In an earlier life I was an engine re-builder and my advice is to scrap the engine block. Don't even think about rebuilding it unless you are prepared to line bore all journals and rebore the cylinders. Even then I would recommend scrapping the block because you will never know if the distortion was the result of excessive heat, or the cause of the heat - until it happens again.

You could have detonation on the cylinder closest to the bearing. Another reason could be harmonic vibration.

Thanks all of you The inputs helped to improve my hypothesis on this issue - Sanjay