BSA Bearings

NOTE: This is an early draft for review purposes only. Take everything with a large grain of salt

This BSA bearing quick reference is inspired by the Norton Owners Club (NSW) Commando bearing guide. While later BSA parts books and manuals provided much more information regarding the bearings fitted than the Norton equivalents, it is still somewhat fiddly to work through the parts books and / or factory workshop manuals to find what bearings are required.

BSA, like most mototcycle manufacturers, fitted a number of different bearing types to the models over the years. Plain bushes and slipper shells tend to be specialist items, but the various ball and roller bearings were off-the-shelf components which should still be available from industrial bearing suppliers.


The aim of this guide is to eventually compile a full list of off-the-shelf bearings and oil seals, both original fitment and modern cross-references) for all post-War BSA models.

Please note that this is (and probably will remain) a work in progress. We try our best, but don't guarantee to get it all correct. Suggestions and corrections will be incorporated as time permits.

Information for this guide has been obtained from BSA manuals, bearing cross-references, Ruper Ratio's Unit Singles book and contributions from other BSA enthusiasts

General bearing stuff

Bearings come in 4 basic forms

• bushes
• slipper shells
• balls
• rollers

bushes are generally machined from sintered bronze, and can be made by any machinist with access to a lathe and supplies of bearing stock. Unless you have ready (cheap) access to a lathe, expertise and sintered bronze rod it is usually best to buy bushes ready-made from a motorcycle parts supplier. Bushes tend to be used in low(er) friction / limited space locations with a reasonable oil supply. Most camshaft and gear pinions run on bushes, as do many gearbox layshafts. Gudgeon pins (wrist pins for the Americans) also usually run on bushes. Bushes are also used for light loads in applications with limited lubrication, such as rocker shafts. Bushes have a high load-carrying capacity

slipper shells are thin steel shells with a lining of softer metal (usually babbit, white metal or aluminium alloy) for the friction surface. The bearing should have no metal-to-metal contact, but relies on an oil film between the disparate metal surfaces. The usual application is for connecting rod big-end shells running on one-piece crankshafts. The infamous BSA twin "timing side bush" is actually a cylindrical slipper shell. Used slipper shells can be re-metalled by specialists, but it is usually best to buy new replacements from a motorcycle parts supplier. Slippers have a high load-carrying capacity and require good high-pressure oil supply

Ball bearings come in 2 forms, loose or caged. Loose balls are generally only used in steering head races or (very early) wheel bearings. The races (cup and cone) are specialist items, but the balls should be avaiilable from bearing suppliers. The ball bearings used most often are ball journal bearings, which come complete with the inner and outer races (cup and cone). The balls are held in place by a cage, which may be made from steel, brass or polyamide. Brass cages tend to be best for engine and gearbox use. Some applications also have a cage without the cup and cone. The BSA 6-spring clutch used a pair o open ball cages.
Ball bearings have a much lower load-carrying capacity than bushes or rollers, but can carry axial loads as well as radial loads. This makes them useful in situations where a shaft has to be held in place with minimal sideways movement. Ball bearings can work in poorly lubricated environments, and sealed pre-lubricated ball bearings are to be preferred ito unsealed versions n exposed applications such as wheel bearings.

Roller bearings can come loose or caged. Rollers also come in various aspect ratios. Long thin rollers are referred to as needle rollers. Rollers can also have various degrees of radius at the ends, ranging from very sharp to rounded.
Roller bearing assemblies are either parallel or taper. Parallel rollers have a very good radial load-carrying capacity but are quite poor for axial loads. Tapered rollers have good radial and axial load-carrying capacity - in *one* direction. They tend to be used in pairs at both ends of a shaft (eg the steering heads of the oil-in-frame twins, car wheel bearings) and need a slight pre-load to stop them chattering. Needle rollers come in parallel or radial configurations. radial needle rollers are used as thrust bearings. They have a very high axial load capacity, but next to no radial capacity.
Roller bearings can work with light lubrication. Sealed pre-lubricated rollers are to be preferred to unsealed rollers in exposed applications

BSA engine and gearbox bearing units generally seem to be C3 clearance (larger clearance than standard). Brass cages are preferred. Brass is comparatively soft, so causes less damage than steel if the cage falls apart. It handles heat better than polyamide so brass-caged bearings can be re-used after removal by heating.

What do all the markings mean?

Bearing assemblies almost always have numbers on one edge of the outer race, inner race or both races . This number should represent the base bearing assembly type, with one or more modifiers. The 1971 BSA twins workshop manual has a section on the modifier markings. Siimilar sheets should be available from bearing companies.
The main modifers of interest for British bikes are:

Imperial and metric dimension

BSA Unit Singles C15, C25 B25, B40, B44, B50

Engine

Gearbox

Wheels

Front end

Rear end

BSA Pre-Unit Twins A7 / A10

Wheels

BSA Unit Twins A50 / A65 / A70

Engine

Gearbox

Wheels

Front end

BSA / Triumph triples A75 Rocket Three, X75 Hurricane, T150/T160 Trident

Engine

Gearbox

Wheels

Steering head

What goes with which?

There are some useful online cross-references as well

NSK bearing cross-reference