Crankshaft thrust bearings provide a fore/aft gap-control for axial movement (or endplay) of the crankshaft. These thrust bearings are located at a specific main bearing location, generally at the center main or rear main, depending on engine design. A thrust bearing is either integrated with a specific main bearing assembly or independent of the main journal bearing. If integrated, the thrust bearing area is present in the form of flanges that extend from the front and rear of the main bearing shells. If independent, the half-moon-shaped thrust bearings are inserted separately into shallow reliefs on the front and rear of the main bearing saddle, and sometimes with the cap.
In either case, the thrust surfaces are located on each side of the designated main bearing saddle and cap, intended to maintain a specified fore/aft location of the crankshaft, so that the crank doesnt walk too far forward or rearward. If theres too little thrust clearance, the thrust bearing surfaces cant maintain a sufficient oil film and will overheat and be destroyed, potentially allowing the crankshafts main journal fillets to walk into the saddle and cap areas, quickly resulting in severe crank damage. If too much clearance exists, the much-needed oil film cant be maintained, eventually leading to thrust bearing failure as the crank is thrust forward during converter or clutch operation, pounding the thrust face and applying unwanted loads on the rod bearings and even piston wrist pin/rod/piston surfaces.
Radial bearing surfaces (main bearings, for example) provide a slight out-of-round taper at the bearing parting line areas to create an oil wedge that supports the crank journals. However, thrust bearing faces are generally flat, facing a flat thrust area on the crank. In order for oil to be delivered to the thrust bearing-to-crank gap, the thrust bearing will generally feature grooves that allow oil from the radial bearing to seep onto the thrust bearing face, providing a film of lubrication. Some thrust bearings now feature tapered edges that help to promote an oil wedge. We see this in some diesel and late-model passenger car engines.
When a thrust bearing failure is discovered, its usually too late. The damage has been done, to the thrust bearing itself and likely to the crankshaft as well. The causes of a thrust bearing failure can be traced to a single problem or a combination of problems.
Naturally, improper assemblyselecting the wrong thrust bearing thickness, contamination during assembly, etc.can lead directly to thrust bearing failure. In general, though, one or more associated problems are usually to blame, including poor crankshaft surface finish, bearing overloading or bearing surface misalignment.
Current transmission designs have been known to contribute to thrust bearing failure due to the greater thrust loadsoften in excess of lbs. Because of the original equipment manufacturers efforts to improve fuel economy and minimize noise, vibration and harshness, many automatic transmissions feature increased forward thrust loads that are delivered through the converter.
In addition, on manual transmission vehicles, the use of starter lockout systems (where the clutch pedal must be depressed in order to start the engine) adds to the problem, since this places forward pressure to the crankshaft during starting, when theres little or no oil lubrication on the thrust bearing surfaces. Bearing manufacturers have tried to address these OE-inherent problems by adding a fine dispersal of silicon to the bearing matrix and by designing angled ramp areas on the thrust faces that help to promote oil delivery to the thrust areas.
The thrust bearing must be able to absorb forward thrust loads that are delivered by the transmission, torque converter or clutch. Thrust bearing overloading can be caused by any number of problems, including poor crankshaft surface finish (too rough and/or wavy), excessive riding of the clutch pedal, improper clutch release bearing adjustment, excessive torque converter pressure or an improperly mounted front crank-driven accessory (a/c compressor, power steering pump, supercharger, etc.).
If a failed thrust bearing is found, dont automatically blame the engine builder. Its very possible that the transmission or a transmission-related area is the culprit. If the action of the clutch, torque converter or automatic transmission hydraulic force applies constant or frequent forward pressure on the crankshaft, certainly the front thrust bearing surfaces are going to take a beating. In the case of a manual transmission, this can be caused by driver error, as a result of continuously riding the clutch. In the case of an automatic transmission, if the converter constantly pushes forward, the same problem will occur.
If the transmission cooler lines have been pinched or crimped, excess pump pressure can cause the converter to act like a hydraulic ram, continuously jamming the crank forward. If the vehicle in question has undergone an engine replacement, make a point to closely examine the transmission cooler lines for any signs of kinking, collapse or other problems.
When beginning to diagnose a thrust bearing failure, instead of suspecting improper engine assembly, first consider the transmission-related issues that can introduce undue forward loading against the crankshaft. Check to see if any external changes were made, such as replacement of or installation of an additional cooler. Ask the customer if any performance modifications were made to the transmission. Verify that the correct flexplate and flexplate bolts were installed, and that the transmission was installed with proper alignment to the engine block (dowel pins in place). If transmission-to-cooler pressure is too high, and the return line pressure is lower, inspect for restricted cooler and cooler plumbing.
In the case of a manual transmission, also check the clutch release bearing for proper adjustment. If the release bearing shows signs of extreme wear and/or overheating, this is a clear indication that the driver has been routinely riding the clutch pedal and/or slipping the clutch.
All too often, performance enthusiasts who encounter thrust bearing failure tend to blame a ballooning torque converter. Yes, a converter body can balloon (expand) under excess pressure, which would force the flexplate and crank forward, but this is rarely the case in a street-driven vehicle. Other converter-related issues should be considered first, such as the wrong flexplate bolts being used, the wrong converter for the application, improper converter installation or the transmission pump gears installed backward.
As CEO Dennis Madden noted in an ATRA report, although all of these problems will cause undue force on the crankshaft thrust surface, they will also cause the same force on the pump gears, since all of these problems will put equal force in both directions from the torque converter. So any of these conditions should also cause serious pump damage very quicklywithin minutes or hours.
Quoting from the ATRA report:
The force on the crankshaft from the torque converter is simple. Its based on the same principle as a servo piston or any other hydraulic component: Pressure multiplied by area equals force.
The pressure part is easy; its simply the internal torque converter pressure. The area is a little more tricky. The area thats 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 often blamed); however, the forward force on the crankshaft occurs because the front of the converter has more surface area than the rear (the converter neck is open). This difference in area is equal to the area of the inside of the converter neck.
As noted in the ATRA thrust bearing report, the most common example is the THM 400 used behind a big-block Chevy. General Motors claims this engine is designed to sustain a force of 210 lbs. on the crankshaft. The inside diameter of the converter hub can vary from 1.50 to 1.64 in. Therefore, the area of the inside of the hub can vary from 1.77 to 2.11 sq. in.; 210 lbs. of force divided by these two figures offers internal torque converter pressures of 119 to 100 psi, respectively.
So, depending on the inside diameter of the hub, it takes between 100 and 119 psi of internal converter pressure to achieve a forward thrust of 210 lbs. The best place to measure this pressure is at the outgoing cooler line at the transmission, because its the closest point to the internal converter pressure. The pressure gauge must be teed in, to allow the cooler circuit to flow. Normal cooler line pressure will range from 50 to 80 psi under a load in Drivefar too low to create a forward thrust of 210 lbs.
If line pressure is unequal and hydraulic force is readily available to push the converter, the simple act of placing the transmission in a forward gear can be enough to jam the crank forward, placing a direct load against the rear thrust bearing.
One step for combating restrictions in the cooler circuit is to run larger diameter cooler lines (to increase volume and reduce pressure). Another is to install an additional cooler in parallel with the original, rather than in series. This will increase cooler flow considerably and reduce the risk of overcooling the oil in winter. The in-parallel cooler may freeze up under very cold conditions; however, the cooler tank in the radiator will allow free flow.
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 create high pressure.
When installing an automatic transmission, pay attention to the basics. Before securing the converter-to-flexplate bolts, check for converter freeplay fore/aft. Typically, the converter should be able to move about 18 to 316 in., to allow for expansion. If this clearance is too tight, the converter will place unwanted forward pressure on the crankshaft, which can wipe out the thrust bearings in a matter of days or even a few hours of driving.
If youre performing an engine build or rebuild, once the crankshaft (along with main bearings and caps) has been fully installed, dont automatically begin to install rods and pistons. Use a dial indicator to check crankshaft endplay and thrust. With the block positioned upside down, place a dial indicator, mounted to a magnetic base stand, onto the block pan rail or other suitable flat ferrous surface. Orient the dial indicator plunger horizontally, parallel to the crankshaft length. Place the plunger against one of the crank counterweights. Using a lever, push the crankshaft fully rearward and preload the indicator by about .050 in., then zero the indicator gauge. Next, push the crankshaft fully forward, noting the amount of movement on the indicator.
Perform this check a few times to verify consistency. Compare your recorded distance to the factory specification range. If too tight or too loose, you may be able to replace the thrust bearing with a proper thickness to achieve the desired specification. Check with your bearing supplier for optional thrust bearing thicknesses.
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Before installing the crankshaft and main caps, be sure to thoroughly lubricate the exposed main bearing and thrust bearing surfaces with a quality engine assembly lubricant or, at the very least, a clean 30-weight nondetergent engine oil.
Instead of simply installing the main bearings and thrust bearings and tightening the main cap fasteners to the factory-specified torque value (or torque-plus-angle value), spend a few extra minutes in an effort to maximize thrust bearing life.
Once all main bearings and caps have been positioned, tighten the main cap bolts (or nuts, in the case of main cap studs) to a minimal value of about 10 to 15 ft.-lbs., following the factory-recommended tightening sequence. This initial snugging of the fasteners will seat the bearings into their respective block and cap saddles.
Next, loosen all of the main cap fasteners, but dont disturb the main caps. Using a rubber or plastic mallet, tap the crankshaft rearward to close up the front thrust clearance. Finger-tighten all main cap fasteners.
Next, use a clean pry bar, such as a long flat-blade screwdriver, to force the crankshaft fully forward. This will help to align the rear thrust bearing faces.
While holding the crankshaft in this pushed-forward position, once again tighten all of the main cap fasteners to 10 to 15 ft.-lbs. You may now release pressure by removing the pry bar. Finish tightening all of the main cap fasteners to their final specification, in two or three equal steps. In the case of a factory specification that calls for a torque-plus-angle tightening, apply the recommended torque value in two steps, followed by the additional angle rotation in two or three steps. Youll need to closely note the number of degrees of rotation during each step to make sure you end up with the total number of degrees of specified rotation.
Taking the extra time to tighten the main bearing caps in this manner will help to align the thrust bearing surfaces with the crankshaft. Aligning the thrust bearings to the cranks thrust surfaces results in maximizing the bearing contact area, which helps to ensure the thrust bearings load-carrying capabilities.
This may sound like a fantasy at first, but its been noted that some thrust bearing failures have been caused, or at least promoted, by inadequate engine grounding. How can a poor ground result in trashing a thrust bearing? When the starter is engaged, the current flow wants to go somewhere. If the engine isnt properly grounded, its entirely possible for the current to run through the crank, and potentially directly into the thrust bearings steel backing. If this occurs often enough, the thrust bearing faces can quickly erode, which then affects the thrust bearing. Its like the thrust surface on the crankshaft isnt finished properly.
How do you check for poor grounding? Following is an explanation of a test taken from a service bulletin written by ATRAs Dennis Madden:
Its easy to check for excessive voltage in the drivetrain: Connect the negative lead of your DVOM to the negative post of the battery, and the positive lead to the transmission. You should see no more than .1 volt on your meter while the starter is cranking.
For an accurate test, the starter must operate for at least 4 seconds. It may be necessary to disable the ignition system so the engine wont start during the test.
If the voltage is excessive, check or replace the negative battery cable, or add ground straps from the engine to the frame, or from the transmission to the frame.
Some systems may reach .3 volt momentarily without incurring a problem. For added assurance, improve the ground with a larger battery cable or additional ground straps.
Although the greatest current draw usually occurs while the starter is cranking, current in the drivetrain can occur while accessories are operating. Thats why you should perform this voltage drop test with the ignition on and as many accessories operating as possible. Again, the threshold is .1 volt.
One final problem that may occur is current though the drivetrain without measurable voltage. If the grounding problem is in the chassis but the engine and transmission grounds are okay (or vice versa), the vehicle may pass the test. What happens here is the ground circuit can be completed through the driveshaft and suspension.
To test this, measure the voltage drop with the driveshaft removed. Both the drivetrain and frame must pass the .1-volt test. This is where a ground strap from the engine or transmission to the frame does its best work.
Diagnosing a thrust bearing failure can sometimes be simple, but often is tricky. Chances are the problem is more likely to occur with an automatic transmission setup. Check torque converter freeplay; beyond this initial check, start checking transmission line pressure and look for kinked and/or restricted cooler lines.
Try to eliminate the transmission-related variables first. Simply rebuilding an engine (and naturally verifying clearances and assembly in the process) and reinstalling it wont cure the problem if the source doesnt lie in the bearing/ crankshaft. If the cause is transmission-related, the thrust bearing will act like a fuse and fail again. In other words, dont simply repair the engine and send the vehicle out the door. The root cause of the thrust bearing failure must be determined and remedied.
Its not actually common for crankshaft thrust bearings to fail but when they do, the result can be catastrophic for the engine. Aside from the significant costs for repair, a thrust bearing failure can pose a serious safety threat to operators in the general area. So, what is the purpose of a crankshaft thrust bearing and why do they sometimes fail? Here we answer your questions and advise on how to prevent an engine disaster.
In an engine block, crankshaft thrust bearings are placed into grooves formed in the blocks webbing. Their purpose is to limit the crankshafts forward and backward movement by keeping the crankshaft within a specific area of motion. By controlling the majority of the forward and backward movements, tolerances within the entire rotating assembly can be preserved which, at high revolutions, is essential for maintaining the engines lifespan.
The three most common causes of thrust bearing failure (and how to avoid them) are as follows:
The presence of dirt and contaminants is damaging to every type of bearing, so its also an obvious cause of failure in thrust bearings. Make sure you have an effective maintenance schedule in place and that cleaning is performed with clean cloths and refined oil. Polishing the surface of the thrust surface, preferably with a polishing machine, is also important. If any grinding marks are left on the crankshaft face, they can act like wiper blades, removing the thrust bearings essential oil film.
Without proper care, cleaning and maintenance, the thrust bearing may become misaligned and begin grinding against a surface, causing sparking. This results in hot spots which can damage the overall operation. Its important to maintain a thrust surface that is set at 90° to the crankshaft centre-line, producing a thrust face that is square to the axis of the main bearing journal.
Overloading is usually caused by either operator error or as the result of a distorted crankcase. In both instances, overloading will cause unnecessary wear and tear, as well as possible thrust bearing failure. To avoid overloading as an operator, its important not to idle excessively or attempt to hot rod. If the problem lies with the crankcase, try to remove any additional loads and repair the bearing using a 90º oil routine.
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