Click on a thumbnail to see the full-size image
Thrust Bearing Failure
By Larry Carley
Thrust bearings are used to control end play in the crankshaft. End play is important because it limits the fore and aft movement of the crankshaft in the block. If an engine is assembled with too much end play in the crank, or if the thrust bearing fails, the forward movement of the crankshaft in the block can chew up the main bearing caps and block. Excessive end play can also cause connecting rods to fatigue and break, and wrist pins to work loose and score the cylinders.
For years engine and transmission rebuilders have struggled to determine the cause of crankshaft thrust bearing failures. Often, each has blamed the other for the resulting damage.
To get to the bottom of the issue, the Automotive Transmission Rebuilders Association (ATRA), the Engine Rebuilders Association (AERA), the Production Engine Remanufacturers Association (PERA), the Automotive Service Association (ASA) and bearing manufacturers got together and came up with a list of possible causes and remedies for thrust bearing failures. Their findings are published in AERA tech bulletin TB-1465R (March 1998).
What They Found
Aside from obvious causes such as dirt contamination and misassembly, there are only three things that generally cause thrust bearing failures.
- Poor crankshaft surface finish;
- Misalignment of the thrust bearing and crankshaft;
- Overloading of the thrust bearing.
Crankshaft thrust faces are difficult to grind because they are done using the side of the grinding wheel. Grinding marks left on the crankshaft face produce a visual swirl or sunburst pattern with scratches, sometimes crisscrossing one another in a cross-hatch pattern similar to honing marks on a cylinder wall. If these grinding marks are not completely removed by polishing, they will wipe the oil film from the surface of the thrust bearing much like multiple windshield wiper blades. A properly finished crankshaft thrust face should only have very fine polishing marks that go around the thrust surface in a circumferential pattern.
Manufacturers of crankshaft micropolishing equipment all say polishing the thrust surface on the crank is just as important as polishing the journals. They also say machine polishing is more accurate and consistent than hand-polishing.
Doug Anderson of Grooms Engines Parts Machining in Nashville, TN, says his company uses a micropolishing machine for polishing crank journals and thrust surfaces. "The problem with polishing cranks manually is that it is totally operator dependent. It’s more of an art than a science and it’s hard to control. Our machine uses a separate polishing arm for the thrust surface. Micropolishing the thrust surface has totally eliminated any thrust bearing failures for us."
Anderson says polishing the thrust surface is especially important on vehicles with electronic torque converters to minimize the risk of thrust bearing failures.
Larry Eriksson of Crankshaft Rebuilders in Sanford, FL, is another who swears by micropolishing thrust surfaces. He says hand polishing is too inconsistent. "The smoother you can make the thrust face, the longer it will live – especially in engines like 302 Fords that have a small thrust face and are prone to thrust bearing failures."
Dennis Janisewski of Gopher Engines in Minneapolis, MN, explains his company has been using a micropolishing machine for about three years. The results, he says, have been "absolutely outstanding." Before micro polishing, Janisewski says the best finish his people could achieve using hand polishing equipment was 10 to 12 microinches. Now, they achieve a finish of six microinches on the thrust surface – which has cut thrust bearing failures "way down."
The thrust surface on the crank must be ground perpendicular to the crank, so close attention needs to be paid to the grinding wheel. The side of the grinding wheel must be dressed at exactly 90 degrees to produce a thrust face that lines up properly with the thrust bearing.
The grinding wheel side face must be dressed periodically to maintain a clean, sharp cutting surface. A grinding wheel that does not cut cleanly may create hot spots on the work piece leading to a wavy, out-of-flat surface. Coolant should be used to minimize heat build-up.
The crankshaft grinding wheel must also be fed into the thrust face very slowly and allowed to "spark out" completely. Also, only a minimal amount of metal should be removed to clean up the surface.
According to the AERA report, the thrust surface on many remanufactured crankshafts often does not require grinding. The crankshaft can be installed with standard bearings or oversize thrust bearings. But if oversized bearings are used, the crankshaft thrust surface must be remachined to compensate for the increased thickness of the bearing. If not, the crank may not have the proper amount of end play.
When the thrust bearing is installed, AERA recommends a several step process to assure proper alignment.
- Tighten main cap bolts to approximately 10 to 15 ft.lbs. to seat the bearings, then loosen.
- Tap main cap toward rear of engine with a soft faced hammer. Then retighten 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 the crankshaft forward, tighten main cap bolts to 10 to 15 ft.lbs.
- Finish tightening the main cap bolts to specifications in two or three equal steps. This should align the bearing thrust faces with the crankshaft to maximize the amount of load bearing area in contact with the crank.
AERA says a number of factors may contribute to wear and overloading of a thrust bearing:
- Poor crankshaft surface finish;
- Poor crank surface geometry;
- External overloading due to:
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.
AERA says people sometimes blame torque converter ballooning, the wrong flexplate bolts, the wrong torque converter, the pump gears being installed backward or the torque converter not installed completely for overloading the thrust bearing. Although all of these conditions will increase loading on the thrust surface, it will also cause the same loading on the pump gears. This can cause serious pump damage within minutes or hours.
By the time a thrust bearing failure becomes obvious, the parts have usually been so severely damaged that it’s hard to tell what caused it. The bearing is generally worn into the steel backing and the crankshaft thrust face is also worn. So how do you tell what happened? AERA says look for the most obvious internal sources.
- Is there any evidence inside the engine of 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 generate enough heat to disrupt bearing lubrication.
Examine the front thrust face on the crankshaft for surface finish and geometry. This may give you a clue as to the original quality of the failed face.
Once you are satisfied that all the potential causes inside the engine have been eliminated, ask about potential external sources of overloading or misalignment. The main one here is the transmission.
- Did the engine have a prior thrust bearing failure?
- What transmission parts were replaced?
- Were 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 properly 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 release bearing properly adjusted?
- What was the condition of the release bearing? A properly adjusted release bearing that is worn or overheated may indicate the driver was riding the clutch. Make sure the clutch has adequate free play.
Vehicles with manual transmissions may also experience thrust bearing failures due to high clutch pressures if someone has installed a "performance" clutch with very high spring pressures.
Who’s To Blame?
The torque converter is often blamed for exerting excessive force against the crankshaft, causing the thrust bearing to fail. There are many theories on this subject, ranging from converter ballooning to spline lock. Most of these theories have no basis in fact, says AERA.
Transmission oil pressure does exert a force that tries to expand the converter like a balloon (which is why converter ballooning is often blamed). The front of the converter has more surface area than the rear because the converter neck is open. This is why the converter pushes forward against the crankshaft. But the amount of force usually isn’t great enough to cause excessive wear on the thrust bearing.
General Motors says the thrust bearing on a big block Chevy is designed to withstand a sustained force of 210 lbs. against the end of the crankshaft. That would require an internal pressure of 100 to 119 psi inside a torque converter – which normally operates at 50 to 80 psi. So the only way the torque converter could cause a problem is if it is experiencing higher than normal operating pressures.
One of two things can cause excessive torque converter pressure: a restriction in the cooler circuit; or modifications or malfunctions in the transmission that increase line pressure.
One way to combat restrictions in the cooler circuit is to run larger cooler lines. Another is to install an additional cooler in parallel rather than in series. This will increase cooler flow considerably and reduce the risk of over cooling the oil during cold weather. The external parallel cooler may freeze up under very cold conditions, but the cooler inside the radiator will still flow freely.
Modifications that can increase 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.
Bearing manufacturers have introduced improved thrust bearing designs to reduce the risk of failure. Thrust bearings rely on an oil film the same as connecting rod and main bearings, but they are not designed to support high loads. Rod and main bearings can support loads measured in thousands of pounds per square inch but thrust bearings can only support a few hundred pounds. The reason why is because thrust bearings are flat and can’t maintain an oil wedge the same way a curved bearing can.
In recent years, bearing manufacturers have switched to "contoured" thrust bearings that can handle higher loads. Some use thrust washers or a three-piece flange assembly. The contact surface has multiple tapered ramps and relatively small flat pads, or a curved surface that follows a sine-wave contour around their circumference. This allows the bearing to maintain an oil wedge and support greater loads.
On Chrysler 3.5L engines, a six-piece thrust bearing is used. The two thrust flanges are loosely held on each bearing with four small tabs so they can move around and align themselves to the crankshaft. They also incorporate a "ramp and flat" design that increases their load carrying capacity by a factor of three compared to that of a conventional, flat-faced, thrust bearing.
Be sure to use an equivalent bearing when rebuilding one of these engines because Chrysler’s FWD transmissions tend to load the thrust bearing pretty heavily. The upper bearing is grooved and the lower one is not.
Some engine builders have resorted to "modifying" thrust bearings when all else fails to cure a thrust bearing problem. One simple modification that can be made to the upper thrust bearing to improve lubrication is to file more chamfer (approximately .040˝ or 1 mm) on the inside diameter edge of the bearing parting line.
The chamfer should be toward the rear thrust face only. 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 preexisting groove to reach the loaded thrust face without passing through the bearing clearance first.