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Servicing a Main Seal

One of the most burdensome engine repairs is a leaky rear main engine seal. To tackle the job, you need to know that there are three types of rear main crankshaft seals used. 

  • Rope/wick seal. This seal got its name from resembling a rope. The material is engineered to withstand the high rpm of the crankshaft while rubbing against it at extreme temperature swings. 
  • Neoprene split seal. This two-piece lip-style seal (shown on the opposite page) is similar to a front timing cover seal. The difference is that the split seal does not use any metal retainer. When looking at a split seal, there will be a lip on one side. The lip must face the crankshaft to seal the oil. If the seal is installed backward, it will leak immediately on engine startup.
  • Neoprene one-piece seal. The one-piece neoprene-style seal is completely round and is installed in similar fashion to a timing cover seal. This style of seal has proven the most effective at being trouble- and leak-free. When it does need to be serviced, either the engine or transmission has to be removed for access.

A small amount of oil is supplied to each style seal via the rear main cap to keep it lubricated so it doesn’t run dry and wear prematurely. Oil is also used as a swelling agent to keep the seal tight against the cap and crankshaft.

Repair strategies
When faced with a rear main seal leak, some choices need to be made. In many applications, the engine will need to come out and possibly have the crankshaft removed, especially with a rope seal.

The proper procedure to install the rope in the cavity in the block and the cap is to roll it into place with a pipe or large dowel. Though tools exist (Chinese fingers and others) to try and pull the seal around, the results are often marginal. You will have much better luck retrofitting a split seal (if one is offered) if you do not want to remove the crankshaft.

There are agents on the market that will temporarily swell the seal and either eliminate or slow the leak sufficiently to make it acceptable to you. Motor oil that is labeled “for high-mileage engines” traditionally has a higher level of swelling agent and may be a good first step before a stop leak product is added to the crankcase. Either product is not a repair.

When installing a rear main seal be sure it’s well lubricated with oil or engine assembly lube, or it will immediately wear and will soon leak.

Engine Man on television
Catch Ray Bohacz’s engine tips on the Machinery Show on RFD-TV every Thursday at 9 p.m. and Sunday at 10 p.m. (Eastern times). Go to for more information.

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