Explained: Triumph & BSA Roller Bearing Clearance

Many British motorcycle enthusiast from the everyday rider, builder, or collector may not be aware of the different internal clearances used on crankshaft bearings.

With that being said, this also applies for all bearings as used on motorcycles, automobiles or even as simple as skateboards. Although some bearings look alike they may have different internal clearances that can not be seen to the naked eye or measured on a gauge - not all bearings are created equal.

Bearing internal clearance plays an important role in our vintage British motorcycle world.

Today's blog post I will be covering Triumph and BSA crankshaft bearing clearances. I will also explain what bearing clearance is, how to identify it, and which bearing you need for your application.

Lets get right to it...

What is bearing internal clearance?

Bearing internal clearance (according to SKF) is "defined as the total distance through which one bearing ring can be moved relative to the other in the radial direction (radial internal clearance) or in the axial direction (axial internal clearance)".

More from SKF - "Sufficient internal clearance in a bearing during operation is extremely important if the bearing is to operate satisfactorily. As a general rule, ball bearings should have an operating clearance (or preload) that is virtually zero".

Clearance is designed within a bearing based on the application. Load, fit, and temperature are all variables when searching for a bearing you need.

Can you measure bearing clearance?

Bearing internal clearance cannot be measured using a caliper or a gauge. It is strictly manufactured with specific specifications. 

Bearing manufactures today will always put a bearings clearance both on the packaging and on the bearing its self.

If you have a bearing with no markings it is safe to say that the bearing would have a CN or a normal clearance rating.

 What happens if you install a bearing with the improper clearance?

Installing a bearing with the improper clearance for a specific application can cause serious damage to your engine which include seizer and vibration.

A bearing with too much clearance can cause vibration both axial and radially.

Bearings installed in an application with too little clearance can cause over heating and damage to the bearings. Typically bearings running tight will not last very long. Bearings can "skid" within the race.

How to identify clearance

 R&M (RHP) roller bearing with two scribe marks - CN condition

Identifying bearing clearance (1900's - 1970's) is a lot different compared to today's standards. Manufactures back then used different standards to show bearing clearance.

This "old way" that I am referring to is specific to bearings as fitted to our old vintage Triumph, BSA, and Norton motorcycles (perhaps others). 

Before throwing your old bearing away it is always a wise idea to inspect all markings to determine the type of bearing that the engine was fitted with.

You can then choose to put the same specification bearing inside as the one you took out. Pictured above is a R & M (RHP) roller bearing in a CN condition as can be seen by the two round dots "OO" that are etched onto the outer bearing race. This specific bearing was fitted to Triumph 650 twins from 1966-1972.

Scribe marks can often be very hard to identify as they can be vary faint, usually holding the bearing in the proper light helps. Older bearings in a condition other then CN will have the same or similar markings depending on brand. Bearings in a C3 condition for example will have marks such as "OOO". C2 bearings will have a single "O" etched.

Bearing clearance works in this order: C3, CN & C2. There are more bearing clearances used but the three mentioned here are the most common. Notice how C3 is first, then CN, and along with C2. C3 is known as a "loose fit" while C2 is know as a "tight fit".

Applications

In 1966 the Triumph & BSA 650 model line up (and A50 500 models) where fitted on the drive side with a roller bearing which replaced the earlier deep groove ball bearing. With both Triumph and BSA experiencing crankshaft ball bearing failures and with the overall engine performance gradually increasing over the years, a roller bearing was fitted.

The drive side of the engine is where most of the load occurs which is why a roller bearing was fitted.

For Triumph 650 the O.E.M. part number for the roller bearing was 70-2879 (E2879) and was used until the end of production in 1983. The problem here is that there are two different clearances that where used in production but where kept under the same part number of 70-2879 (E2879).

Below is a chart for Triumph and BSA twins in which we made to show you which bearing clearance you should use for your application.

Final points

Remember, if the factory installed it, they did it for a specific reason. If your not sure, a good rule of thumb is to replace your bearing with the same bearing that you have removed from your application. I hope this blog has made you aware and gave you some insight about bearing internal clearances. 

If you have any questions, comments or you would like to chime in please feel free to drop us a comment in the section below.

Thanks for reading!