For years both transmission and engine rebuilders have struggled at times to determine the cause of crankshaft thrust bearing failures.
In most instances, all of the facts concerning the situation are not revealed at the onset of the failure. This has led to each party blaming the other for the failure based only on hearsay or what some “expert” has termed the “cause”
Some of those explanations have led to an argument, that ends up in litigation while the truth lingers uncovered in the background.
This document is a group effort of combined information compiled by the Automotive Transmission Rebuilders Association (ATRA), the Automotive Engine Rebuilders Association (AERA), the Production Engine Rebuliders Association (PERA), the Automotive Service Association (ASA) and bearing manufacturers.
This group of industry experts has assembled the following information to consider and offers solutions that may prevent a similar thrust bearing failure.
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. While radial bearings can carry loads measured in thousands of pounds per square inch of projected bearing area, 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. 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.
This flatness makes it more difficult to create and maintain an oil film. As an example; if two gauge blocks have a thin film of oil on them, and are pressed together with a twisting action, the blocks will stick together. This is similar to what happens when a thrust load is applied to the end of a crankshaft and oil squeezes out from between the shaft and bearing surfaces. If that load is excessive, the oil film collapses and the surfaces want to stick together resulting in a wiping action and bearing failure.
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.
In the past few years some new automotive engine designs include the use of contoured thrust bearings to enable them to carry higher thrust loads imposed by some of the newer automatic transmissions. Because it’s not practical to incorporate contoured faces on one piece flanged thrust bearings, these new engine designs use either separate thrust washers or a flanged bearing which is a three piece assembly.
Cause of failure:
- Poor crankshaft surface finish
- Surface finish:
When assembling thrust bearings:
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) Excessive Torque converter pressure.
b) Improper throw out bearing adjustment.
c) Riding the clutch pedal.
d) Excessive rearward crankshaft load pressure due to a malfunctioning front mounted accessory drive.
Diagnosing the problem:
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, and were they installed correctly?
If the thrust bearing is in an end position, was the adjacent oil seal correctly installed? 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.
- Once you are satisfied that all potential internal sources have been eliminated, ask about potential external sources of either over loading or misalignment.
Transmission related problems:
Did the engine have a prior thrust bearing failure?
What external parts were replaced?
Were there any performance modifications made to the transmission?
Was an additional cooler for the transmission installed?
Was the correct flexplate used? At installation there should be a minimum of 1/16″ (1/8″ preferred, 3/16″ maximum) clearance between the flex plate and converter to allow for converter expansion.
Was the transmission property aligned to the engine?
Were all dowel pins in place?
Was the transmission-to-cooler pressure checked and found to be excessive? If the return line has very low pressure compared to the transmission-to-cooler pressure line, check for a restricted cooler or cooler lines.
If a manual transmission was installed, was the throw out bearing properly adjusted?
What condition was the throw out bearing in? A properly adjusted throw out bearing that is worn or overheated may indicate the operator was “Riding The Clutch”.
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 (which is why converter ballooning is probably often blamed), however, it is the fact that the front of the converter has more surface area than the rear (the converter neck is open) that causes the forward force on the crankshaft.
This difference in area is equal to the area consumed by the inside of the converter neck.
The most common scenario is the THM 400 used behind a big-block Chevy. General Motors claims that this engine is designed to sustain a force of 210 pounds on the crank shaft. The inside diameter of the converter hub can vary from 1.5 inches up to 1.64 inches. The area of the inside of the hub can then vary from 1.77 square inches to 2.11 inches. 210 pound of force, divided by these two figures offers an internal torque converter pressure of 119 psi to 100 psi, respectively.
That is to say, that depending on the inside diameter of the hub, it takes between 100 to 119 psi of internal converter pressure to achieve a forward thrust of 210 pounds. The best place to measure this pressure is the out-going cooler line at the transmission because it is the closest point to the internal converter pressure available. The pressure gauge must be “teed” in so as to allow the cooler circuit to flow. Normal cooler line pressure will range from 50 psi to 80 psi , under a load in drive.
Causes for excessive torque converter pressure:
This will increase cooler flow considerably. An additional benefit to running the cooler in parallel is that it reduces the risk of over cooling the oil in the winter time—especially in areas where it snows. The in-parallel cooler may freeze up under very cold conditions, however, the cooler tank in the radiator will still flow freely.
Modifications that can result in higher than normal converter pressure include using an overly-heavy pressure regulator spring, or 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.
If a problem application is encountered, every effort should be made to find the cause of distress and correct it before completing repairs, or you risk a repeat failure.
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. This additional source of oiling will reach the loaded thrust face without passing through the bearing clearance first (direct oiling).
Since there may be a load against the rear thrust face, oil flow should be restricted by that load and there should not be a noticeable loss of oil pressure. This modification is not a guaranteed “cure-all”. However, the modification should help if all other conditions, such as surface finish, alignment, cleanliness and loading are within required limits.
Other External Problems. Aside from the items already mentioned, there is another external problem that should be considered. Inadequate electrical grounds have been known to exacerbate thrust surface wear. Excessive current in the vehicle drive train can damage the thrust surface. It affects the thrust bearing as though the thrust surface on the crankshaft is not finished properly finished (too rough).
Excessive voltage in the drive train can be checked very easily. With the negative lead of a DVOM connected to the negative post of the vehicle battery and the positive lead on the transmission, there should be no more than .01 volts registering on the meter while the starter is turning over the engine. For an accurate test, the starter must operate for a minimum of four seconds without the engine starting.
You should disable the ignition system before attempting this test. If the voltage reading observed is found to be excessive, add and/or replace negative ground straps from the engine to the vehicle frame and transmission to frame until the observed voltage is .01 volts or less. Note: Some systems may show a reading of .03volts momentarily but yet not exhibit a problem. For added assurance, it is a good idea to enhance the drive train grounding with larger battery cable.
Special Thanks to Lance Wiggins and ATRA for Great Information