Servicing a rear main seal.

There is not an engine oil leak more frustrating than a rear main seal. In addition, these are the most burdensome to repair.

There are three types of rear main crankshaft seals in use.

Rope/wick seal: The seal got its name from its appearance that resembles 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: Made from a rubber-like material this is a two-piece lip-style seal. Its design is like a front timing cover seal. The difference being 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 from leaking. If the seal is installed backwards it will leak immediately on engine start up.

One piece: This is a neoprene-style seal that 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. But when it does need to be serviced, either the engine or transmission must 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 that it does not run dry and wear prematurely. Oil is also used as a swelling agent to keep the seal tight against the cap and crankshaft.

Leak repair

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, often the results are 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 one is not a repair.

When installing a rear main seal it must be well lubricated with oil or engine assembly lube or it will immediately wear and leak soon thereafter.