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
- Poor crankshaft, surface finish.
- Surface finish.
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.
Crankshaft Thrust Bearing; Assembly
- 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.
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.
Crankshaft Thrust Bearing Failure; Diagnosing The Problem
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
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
- Restrictions in the cooler circuit.
- Modifications or malfunctions that result in, high line pressure.
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.
Special Thanks to Lance Wiggins and ATRA for Great Information
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