One of the most difficult situations that performance automotive engine builders encounter is crankshaft thrust bearing failure. In many cases this can result in customers and engine builders at odds over just who is responsible. This is understandable if the failure occurred in a newly rebuilt engine. However, in the majority of cases the blame can be placed on other drivetrain components.
As with any bearing in the engine, a small film of oil actually forms between the bearing surface and rotating engine part. The same applies to thrust bearings, which are responsible for maintaining end play which is within acceptable tolerances. Unlike main and rod bearings, the thrust bearing is unable to support the same amount of load. As measured, a rod bearing can support thousands of pounds per square inch while a thrust bearing can only support a few hundred pounds per square inch.
Many industrial engines, which are subjected to extreme conditions, actually utilize thrust bearing spacers. These spacers are designed to accept heavier loads and are made to extend the overall life of the engine. However, spacers are not readily used in high performance engines where extreme use generally involves greater RPM’s instead of added end play force.
When a thrust bearing fails in an engine, and causes damage to the crankshaft, there are typically two conditions which are responsible for this. The first being main cap thrust alignment. This typically occurs if the thrust bearing is not properly fitted to the crankshaft. This is a rare occurrence with any competent engine builder. A sign that the thrust was not aligned properly would be uneven wear on the thrust bearing and possibly visual signs of the main cap walking.
The most common cause of crankshaft thrust bearing failure is an overloading condition. Normally what causes overloading to occur is a failed torque converter which places too much load on the crankshaft. Another condition which creates an excessive load on the crankshaft is the vehicle owner riding the clutch, when a manual transmission is used.
When a torque converter fails, either the cooler has become restricted or excessive high oil line pressure exists. External modifications can lead to torque converter problems, but in many cases either the transmission oil was running too hot or the torque converter has completely failed. Running high performance transmission cooler lines will allow for a lower operating temperature. Regardless, the torque converter should be properly inspected and replaced, if needed. In the cases of crankshaft thrust bearing failures on manual transmission vehicles, in most cases the operator needs to change their driving habits so that the problem does not occur in the future.
While it is understandable that vehicles owners are upset when a crankshaft thrust bearing fails, ultimately the condition that caused the failure was not related to the engine at all. In most cases, the problem was caused by a transmission component. While some performance vehicle owners may point the finger of blame towards their engine builder, the reality is that crankshaft end play is controlled externally by a transmission component.
This publisher is an ASE Certified Engine Machinist for the Crankshaft Company. As a machinist, he is responsible for crankshaft grinding, welding and straightening operations which are routinely performed in his automotive machine shop.