Main Bearing


Out-of-Round Journal Wear

When the engine is first turned over after the engine has not been run for a period of time, there is little or no lubrication between the crank and the lower main bearings. The result is that the lower main bearing wears excessively and the main journals wear out-of-round. When the main bearing that is farthest from the oil pump shows more wear than the other main bearings, a dry start condition is indicated. This means that the engine was probably revved before oil had filled the system. This problem occurs most often in cold weather.

Sometimes, all the lower main bearings will be worn except for the front bearing. This bearing usually wears less on the bottom because of the upward tension of the fan belt. Excessive belt tension can cause wear on the upper front main bearing. Connecting rod journals also wear out-of-round, wearing on their top sides because of excessive loads during the power stroke. Crankshaft main bearing journals wear out-of-round. Excessive loads cause the oil film to break down, resulting in wear. Excess loads can be caused by lugging the engine or by abnormal combustion.

Lugging occurs when the load on the engine is greater than the rpm needed to develop enough horsepower to pull the load.

Measure the rod journal in a horizontal and vertical direction to check for out-of-round wear. Crank journals are miked (measured with a micrometer) at 90° angles to check for out-of-round wear, which should be less than 0.0005".

Tapered Wear

Rod journals sometimes suffer taper wear due to misalignment of the connecting rod.

The presence of uneven rod bearing wear, and sometimes piston skirt wear usually indicates taper. Connecting rods should be checked for misalignment whenever uneven wear is found.

Thrust Bearing Wear

The thrust bearing surface that faces the rear of the engine sometimes shows excessive wear.

One side of this thrust bearing is burned. Courtesy of Federal-Mogul Corporation.

Most thrust bearings have concaved reliefs cut into them to provide lubrication. Under normal conditions, the thrust surface is only under load when the clutch pedal is depressed or if the automatic transmission torque converter is under a load. Thrust bearing wear and failure occurs when the load is continuous, such as when there.

To measure the bearings for wear, taper and out-of-roundness:

  • Using a service manual, look up the specifications for standard crankshaft size and tolerances for normal wear.
  • Check the crankshaft and use proper procedures to clean it before beginning the measurement process.
  • Using the proper size outside micrometer, check the number 1 main bearing journal twice at each end of the journal, once horizontal to the crankshaft and once vertical.
Measuring the connecting rod journal with a micrometer.
  • If these measurements are different than the vehicle's specifications, the crankshaft main bearing journal is out-of-round or tapered and the crankshaft must be machined before it is installed in the engine.
  • If any of the main bearing journals are out of specification, all main bearing journals should be ground to the next undersize. This ensures that the journals are on the same centerline. Another alternative is to build up the crankshaft journal using special welding techniques, then grinding the journal to its original size.
If the main bearing or connecting rod journals have pits, yet measure within specifications, polish the worst journal. After the journal is clean of pits, remeasure it. If it is still within specifications, the crankshaft will not require grinding.
Check for out-of-roundness and taper. Courtesy of Dana Corporation.
  • Repeat these steps for each of the remaining main bearing journals.
  • Measure the number 1 connecting rod journal twice at each end of the journal, once horizontal to the crankshaft and once vertical.
  • If these measurements are different, the crankshaft connecting rod journal is out-of-round or tapered, and the crankshaft must be machined before it is reinstalled in the engine.
  • Repeat these steps for each of the remaining connecting rod journals.
Not all do-it-yourselfers will have the tools necessary to perform these jobs. They can be completed in a machine shop.

Journal Grinding

The manufacturer often applies a hardening treatment to the journals to protect them from wear. However, even with hardened surfaces, the bearing journals can become scored or scuffed due to improper lubrication, excessive heat, contamination, or improper installation. It may be possible to restore the journal surface by grinding them to a standard undersize. Grinding of the crankshaft journals is done to correct any of the following conditions:

  • Out-of-round
  • Taper
  • Improper oil clearances
  • Scratches, scoring, or nicks
  • Damaged thread surfaces

Before grinding the journals, attempt to determine the hardness of the journal. Hardness values are expressed using the Rockwell C (Rc) scale. Electronic hardness gauges are available to determine the journal's Rc value. Nitral acid etching is another method of determining the hardness of the journals. Generally, crankshafts with fillet-hardened journals should have values above 36 Rc. Crankshafts without fillet-hardened journals should have values above 30 Rc.

Crankshaft journals hardened using Tufftriding or Melonite treatments (both are forms of saltbath nitriding) cannot be machined. These processes are too thin, and machining the journals will remove all hardening. To determine if these hardening processes were used on the crankshaft, file a small portion of the counterweight using a medium-fine mill file. If metal can be removed under light filing pressures, the crankshaft has not been treated using these methods.

As with most machining operations, when grinding the crankshaft, do not remove any more metal than necessary. Generally, journals are ground to undersizes of 0.010, 0.020, or 0.30 inch. not all rod journals need to be the same undersize. It may be necessary to machine only one rod journal to an undersize while the others remain standard. However, it is recommended that the main bearing journals be machined to the same undersize to be sure the centerline is on the same plane.

Whenever the crankshaft journals are undersize, it is a good practice to stamp the size of the rod and main journals on the face of the first counterweight. If the main bearing journals are ground to different undersizes than the rod journals, list the main journals first. For example, if the main journals are undersized 0.020 inch and the rod journals are undersized 0.030 inch, the marking will be made as 0.20-0.30. This will alert the next technician rebuilding the engine (or using the crankshaft) that the crankshaft has been undersized.


The machining of the journals and seal surfaces of the crankshaft leaves these surfaces too rough to run bearings or seals on. After the journals are ground, they must be polished to remove this roughness. Generally, the crankshaft is rotated in the opposite direction during the polishing procedures than it was run during the grinding procedure.

Polishing is not a final sizing operation. The maximum amount removed by this procedure should not exceed 0.0002 in. (0.005 mm). The polishing procedure can be done while the crankshaft is attached to the grinding machine using a portable polisher. In addition, a special crankshaft polishing machine can be used or the journals can be polished by hand lapping.

Both types of machines work similarly, using a belt sander-type setup to polish the journals. The crankshaft is rotated and the belt is run back and forth across the journal. Continue to polish the journal until it is smooth and shiny. A surface of 32 in. on the bearing journals and 15 uin. on the bearing journals and 15 uin. On the sealing surface is usually desired. Hand lapping is done using a piece of emery cloth wrapped around the journal. Start with a medium grit cloth and finish with a 320-grit cloth. A finish of 15 uin. is usually obtained by hand lapping.

After the journals and sealing diameters are polished, the blend radius of the oil holes must be deburred and rounded. Polishing the oil holes prevents early bearing failure by removing the sharp corners. This operation can be done using a die grinder or jeweler's rouge.

When the polishing procedure is completed, the crankshaft must be thoroughly cleaned any residue from the grinding and polishing procedures left on the crankshaft or in the oil passages will quickly destroy the journals.

Building Up Crankshaft Journals

Another available option for reconditioning the crankshaft journals is to build up the journal area, then machine it to standard size. This process is used when the journal is worn or damaged so excessively that no undersized bearing can be used to correct oil clearance. There are two common methods used to build up the journals: chromium plating and submerged arc welding.

The process of chromium plating electrically plates hard chromium onto the bearing journal surfaces. After the plating is built up to the required amount, the journals are ground to restore the original size.

A crankshaft welder uses a wire feed-type welding system using flux to displace oxygen. Before the beads are welded to the journal, the journals are ground to remove impurities in the surface area. Carbon plugs are tapped into the oil passages to prevent them from being filled.

The crankshaft is attached to the welding machine between two chunks and is rotated while the weld bead is spiraled around the journal. The process continues until the weld bead has worked across the entire width of the journal. After the welding process is completed, the slag is removed and the journal is ground. Welding to build up the journals provides a very hard surface.