Replacing Bearings and Seals in Motorcycle Cases

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Replacing Bearings and Seals in Motorcycle Cases

A) Warming the case with boiling water. B) The case supported on wood. C) Drifting the bearing out. D) The case ready for a new bearing and seal. John H Glimmerveen Licensed to About.com

During a motorcycle engine rebuild, it is good practice to replace most of the bearings and all of the oil seals.

Most bearings inside an engine are of the ball or roller type and with correct lubrication will last many hours or miles. However, crank bearings – especially on 2-strokes – are subject to high stresses, and if the engine is being rebuilt/refreshed it is an ideal time to replace them. Oil seals are relatively inexpensive and should never be reused.

Of primary importance with crankshaft bearings is their fit inside the parent case. If the bearing is loose inside the case, it will not support the crank properly, which will lead to premature failure of the bearing and/or the crank. Although this situation is rare, should the mechanic find this to be the case, he or she must take the case to a specialist engineering shop for repairs (typically requiring welding and re machining). However, the cases will be damaged if the correct procedures are not followed when replacing the bearings.

Note: Although obvious, it must be remembered that steel is stronger than aluminum and the steel cage of a bearing can easily damage an aluminum case.

A Worked Example

The bearing and oil seal being considered here is located on a Triumph Tiger 90/100 crank case (left side). Although the bearing and oil seal appeared to the mechanic to be in good condition, this particular machine had sat for more than 20 years before being restored, and as such, a small amount of rust was likely in the bearing. This rust could easily work its way around the engine and cause damage to vulnerable items such as the connecting rod’s shell bearings. As the oil seal had to be removed, it, too, would be replaced for safety’s sake.

Before attempting to remove either the bearing or the oil seal, the mechanic should prepare the work area and tools required. Of utmost importance is ensuring the safety of the crankcases, as these are made of cast aluminum and can easily be damaged. In this case the mechanic has placed pieces of wood (pine) to support the case—see photograph.

To remove the bearing, a suitable drift or extractor will be required. In the absence of a proprietary bearing extractor, a socket of the appropriate size will suffice as a drift.

Warming the Case

The case will need to be heated to expand it away from the bearing which will make it easier to drift out. As aluminum expands faster than steel, applying heat to the general area is acceptable. A number of options are available including boiling water, using a gas powered flame (blow torch), and using an electrical oven. The mechanic in this instance chose to use boiling water. However, great care must be taken to avoid burns.

The case was placed over a large bucket and boiling water was then poured over the area surrounding the bearing. A full kettle of water will be required to get sufficient heat into the cases.

If using this method, while waiting for the case to absorb the heat, you should position it on wooden supports. Next, drift the bearing from its location in the case. Once the bearing has been removed, the case can be inverted and the process repeated to drift out the oil seal (if this is done quickly, there will be no need to reheat the case).

Typically the bearing location in the case will need to be thoroughly cleaned, which is best accomplished with the use of fine grade Scotch-Brite applied by hand; however, it is best to degrease the location with brake cleaner first. Before the mechanic begins cleaning the case, it is good practice to prepare the new bearing for assembly by placing it inside a sealable plastic bag then placing that into a freezer. Typically, a crank bearing left inside a freezer will shrink approximately 0.002” (0.05-mm) over half an hour.

Once the area has been cleaned, the case should be reheated. A bearing retaining compound such as Loctite® 609™ (green) should be applied inside the case at the base of the bearing. Only a small of amount of this compound is required. As soon as the compound has been applied, the mechanic should press fit the new bearing.

The amount of pressure required to push the new bearing into the case will be different for every engine; however, a good indication of the amount of pressure needed will be had from the pressure needed to push the old bearing out. Once the new bearing has been located, any excess locking compound should be wiped off before the new oil seal is pressed into position.

Notes:

1) It is imperative that the bearing is pushed into the case in a straight line.

2) Both the new bearing and the oil seal should be pressed into the case by applying pressure to their outer edge. A round object (such as a socket) should be slightly less in diameter than the O/D of the bearing or seal. The mechanic must never press a bearing via its center as this can separate the bearing.

Further reading:

Replacing Wheel Bearings