Changing an oil seal demands attention to detail

Motoring Reporter

Motoring Reporter

You have obtained a new oil seal to replace the old, leaking one.

You have checked that it matches the old one, which is probably somewhat mangled after the struggle to get it out.

Now it's simply a matter of knocking the new seal in where the old one came out. What can go wrong with that?

A few things, unfortunately. To begin with, you should have a good look at the surface of the shaft on which the old seal was riding. Examine the track of the old seal, shown by a shiny ring around the shaft.

Any trace of roughness in that area should be polished out with 1000 grit emery paper soaked in paraffin.

Sometimes, however, there is more than roughness; seals that have been in place for a long time, especially on a vehicle used in dusty conditions, can wear a tiny groove where they contact the shaft.

If that has happened, it's a waste of time to fit a new seal which rides exactly in the track of the old one - it will soon leak again.

Feel carefully with your fingernail, and if you can feel the slightest groove in that shiny ring, break for a cup of coffee while you make a plan.

You have various options. The first is to fit the new seal in such a position that its contact line is on a slightly different part of the shaft - even a millimetre difference is sufficient. This can sometimes be done by fitting a thin shim between the seal and the shoulder of the housing against which it abuts.

The second option is to take the shaft to an engineering shop and ask them to grind it down in order to remove the groove.

If the groove is deep, metal will first have to be added and the shaft must be ground down to restore the original diameter.

This is an expensive and time-consuming option. The third option, which is also expensive, but less time-consuming, is to fit a thin-walled sleeve over the grooved section of the shaft and let the new seal run on the sleeve. Such a product is available in a wide range of diameters from one of the biggest bearing manufacturers, under the proprietary name Speedi-Sleeve.

Made from high-grade stainless steel and having a superior finish on its outside surface, the sleeve is simply pushed in position over the worn area, providing a sealing surface as good as, if not better than, a new shaft.

Its wall is only a quarter of a millimetre thick, thin enough that the same size seal as the original can be used. Once the shaft has been sorted out, you can install the new seal. Put some light grease on the outside of the seal and in the bore of the housing.

Ensure the seal goes in with its "open" side, on which the little garter spring is visible, facing the oil. Always start it by hand, taking care that it doesn't go skew. If you are lucky, you can push it in all the way by hand, applying pressure evenly round the edge, never on the central, flexible part.

Usually you will have to knock it in gently with a hammer. Use a drift to avoid knocking directly on the seal. A large socket, marginally smaller than the outside diameter of the seal makes a good drift as does a flat piece of wood if there's not a shaft protruding from the housing.