Crankshaft Thrust Bearing Failure

For years both transmission and engine rebuilders have struggled at times; to determine the cause of crankshaft thrust bearing failure.

Diagnosing a crankshaft thrust bearing failure can sometimes be simple, but often is tricky. And, chances are the problem is more likely to occur, with an automatic transmission setup.

When a crankshaft thrust bearing failure is discovered, it’s usually too late. Because, the damage has been done to the thrust bearing itself and likely, to the crankshaft as well.

The causes of a crankshaft thrust bearing failure can be traced, to a single problem or a combination of problems.

Crankshaft Thrust Bearing Failure; History

Although thrust bearings run on a thin film of oil, just like radial journal (connecting rod and main) bearings; they cannot support, nearly as much load. So, radial bearings can carry loads, measured in thousands of pounds per square inch of projected bearing area. But, thrust bearings can only support loads of, a few hundred pounds per square inch.

Radial journal bearings develop their higher load capacity; from the way the curved surfaces of the bearing and journal meet to form a wedge. Furthermore, shaft rotation pulls oil into this wedge shaped area of the clearance space; to create an oil film, which actually supports the shaft. Thrust bearings typically consist of two flat mating surfaces; with no natural wedge shape in the clearance space; to promote the formation of an oil film, to support the load.

For this reason, many heavy-duty diesel engines use; separate thrust washers, with a contoured face; to enable them to support higher thrust loads. These thrust washers either have multiple tapered ramps and relatively small flat pads; or they have curved surfaces, that follow a sine-wave contour around their circumference.

Causes Of Crankshaft Thrust Bearing; Failure

So, there are only four common factors which generally cause crankshaft thrust bearing failures:

Poor crankshaft, surface finish.
Misalignment.
Overloading.
Surface finish.

Most of the time crankshaft thrust faces are difficult to grind. Because, they are done, using the side of the grinding wheel. Consequently, grinding marks left on the crankshaft face; produce a visual swirl or sunburst pattern with scratches. Also, crisscrossing one another in a cross-hatch pattern; similar to hone marks on a cylinder wall.

So, if these grinding marks are not completely removed by polishing; they will remove the oil film from the surface of the thrust bearing. A properly finished crankshaft thrust face should only have, very fine polishing marks; that go around the thrust surface, in a circumferential pattern. Additionally, shot peening can dramatically increase a crankshaft’s life; it may be a good idea to look into this.

Alignment

So, a grinding wheel that does not cut cleanly, may create hot spots on the workpiece; leading to a wavy, out-of-flat surface. The side of the wheel must be dressed, at exactly 90° to its outside diameter. This will produce a thrust face that is square to the axis of the, main bearing journal. So, the crankshaft grinding wheel must be fed into the thrust face very slowly; and also allowed to “spark out” completely. The machinist should be very careful to only remove minimal stock for a “clean-up” of the crankshaft surface.

In most instances a remanufactured crankshaft, does not require grinding of the thrust face(s). As a result, the grinding wheel will not even contact them. Furthermore, oversize thrust bearings do exist.

Grinding crankshaft thrust faces, requires detailed attention during the procedure. For instance, repeated wheel dressings may be required. Also, maintaining sufficient coolant between the grinding wheel and thrust surface must be attained; to prevent stone loading and “burn” spots on the thrust surface. All thrust surface grinding should end in a complete “spark out”; before, the grinding wheel is moved away from the area being ground. Finally, following the above procedures with care should; maintain a thrust surface that is 90° to the crankshaft centerline.

Crankshaft Thrust Bearing; Assembly

Tighten main cap bolts to approximately 10 to 15 ft.lb. to seat bearings; then loosen.

Tap main cap toward rear of engine, with a soft faced hammer.
Tighten main cap bolts, finger tight.
Using a bar, force the crankshaft as far forward in the block as possible; to align the bearing rear thrust faces.
While holding shaft in forward position, tighten main cap bolts to 10 to 15 ft. lbs.
Complete tightening main cap bolts to specifications, in 2 or 3 equal steps.

The above procedure should, align the bearing thrust faces with the crankshaft; to maximize the amount of bearing area in contact for load carrying.

Overloading

A number of factors may contribute to wear and overloading of a thrust bearing; such as:

Poor crankshaft surface finish.
Poor crankshaft surface geometry.
External overloading due to.
Excessive Torque converter pressure.
Improper throw out bearing adjustment.
Riding the clutch pedal.
Excessive rearward crankshaft load pressure; due to a malfunctioning front mounted accessory drive.

Note: There Are Other Commonly-Thought Issues; Such As:

Torque converter ballooning.
The wrong flex plate bolts.
The wrong torque converter.
Pump gears being installed backward.

All of these problems will cause, undue force on the crankshaft thrust surface. It will also cause the same undue force on the pump gears; since all of these problems result, in the pump gear pressing on the crankshaft; via the torque converter. The result is, serious pump damage in a very short period of time (within minutes or hours). So, to help prevent any further damage; you should make sure to look after your, torque converter as well.

Crankshaft Thrust Bearing Failure; Diagnosing The Problem

So, by the time a crankshaft thrust bearing failure becomes evident; the parts have usually been so severely damaged that there is little, if any evidence of the cause. Furthermore, the bearing is generally worn down to the steel backing. And, has severely worn the crankshaft thrust face as well. So, how do you tell what happened? Start by looking for the most obvious, internal sources.

Engine Related Problems

Is there evidence of distress anywhere else in the engine; that would indicate, a lubrication problem or foreign particle contamination:

Were the correct bearing shells installed, correctly?
If the thrust bearing is in an end position, is the adjacent oil seal installed correctly? An incorrectly installed rope seal, can cause sufficient heat to disrupt bearing lubrication.
Examine the front thrust face on the crankshaft for surface finish and geometry. This may give an indication of the original quality of the failed face.

Crankshaft Thrust Bearing Failure; Transmission Related

Did the engine have a prior, crankshaft thrust bearing failure:

Were there any external parts replaced?
Did the transmission have any performance modifications done to it?
Was a transmission cooler added?
Correct flex plate used?
Was the transmission and engine properly aligned?
Were all dowel pins in place?
Was there excessive pressure from the transmission-to-cooler? If the return line has very low pressure compared to the transmission-to-cooler pressure line; check for a restricted cooler or cooler lines.
What condition was the throw out bearing in?

So, How Does The Torque Converter, Exert Force On The Crankshaft

There are many theories on this subject, ranging from converter ballooning to spline lock. But, most of these theories have little real bases and rely little on fact. The force on the crankshaft from the torque converter is simple. It is the same principle as, a servo piston or any other hydraulic component. Pressure, multiplied by area, equals force. The pressure part is easy, it’s simply the internal torque converter pressure.

The area is a little trickier. The area that is part of this equation; is the difference between, the area of the front half of the converter and the rear half. The oil pressure does exert a force, that tries to expand the converter like a balloon. However, it is the fact that the front of the converter, has more surface area, than the rear; that causes the forward force on the crankshaft.

Causes For; Excessive Torque Converter Pressure

So, there are two main causes for excessive torque converter pressure:

Restrictions in the cooler circuit.
Modifications or malfunctions that result in, high line pressure.

One step for combating restrictions in the cooler circuit is to, run larger cooler lines. Another, is to install any additional cooler in parallel, as opposed to in series.

Modifications that can result in higher than normal converter pressure include:

Using an overly-heavy pressure regulator spring.
Excessive cross-drilling into the cooler charge circuit.

Control problems such as, a missing vacuum line or stuck modulator valve; can also cause high pressure.

Always find the cause of distress and correct it, before completing repairs; or you risk a repeat failure.

A simple modification to the upper thrust bearing, may be beneficial in some engines. Install the upper thrust bearing in the block to determine; which thrust face is toward the rear of the engine. Using a small, fine tooth, flat file, increase the amount of chamfer to approximately .040″ (1 mm); on the inside diameter edge of the bearing parting line. Finally, carefully file at the centrally located oil groove; stroke the file at an angle, toward the rear thrust face only.

Above all, it is very important not to contact the bearing surface with the end of the file. The resulting enlarged ID chamfer will allow; pressurized engine oil from the pre-existing groove to reach the loaded thrust face. As a result, this additional source of oiling will reach the loaded thrust face; without passing through the bearing clearance first (direct oiling).

Conclusion

Special Thanks to Lance Wiggins and ATRA for Great Information

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