THE PLAY’S THE THING

Q In last month’s Ask

Our Experts, you had a brief piece on choosing bearing clearances. As a newcomer to classic cars, it didn’t mean much to me. Can you explain in more detail?

Antonia Pratt, Hull

A A bearing consists of an outer and an inner track with balls or rollers in between. The balls are very slightly smaller than the space between the tracks, which allows free movement of any part of the bearing relative to any other part. It also allows space for a layer of lubricant to remain between the moving tracks and balls, which would otherwise be pushed out of the way.

This clearance introduces an element of ‘rock’ into the bearing. This may be undesirable – for example, a car wheel that can be rocked in and out significantly would fail an MOT. However, if the bearing operates in a hot place, such as inside an engine, a greater clearance is required to allow for expansion.

Bearings are therefore made with a variety of clearances to suit their working temperature and the degree of precision required. A C1 bearing is very precise. A C2 has a standard clearance suitable for most car applications where heat is not a major factor. A C3 has a wider clearance when cold, becoming normal when hot – so it’s ideal for water pumps, gearboxes, etc. A C4 bearing might be found in a highly-tuned two-stroke motorcycle engine.