Tech Tip: Solid Bronze Bushings vs Steel / Bronze Inserted Bushings

As with anything else that is discussed on the internet, the debate regarding BSA and Triumph crankshaft bushings is alive and well.

Even today, 2019..

You may have come across some popular threads on Britbike forum regarding BSA crankshaft bushing materials and which to install.

Here are some of the popular threads... (copy and paste into browser) 

• http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=354679

• http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=703712&page=1

The links above clearly showcases a few members among the Britbike.com community voicing there own opinions on a simple yet confusing topic.

Here at CBS we get our own share of questions revolving around this topic such as:

• “Do I need a solid bushing or a steel bushing for my crank?”
• "Which bushing will last longer? - a steel busing or a solid bronze bushing"?
• "Which bushing is easier to install?"

Today I have decided to set off on a clear and practical answer to help you figure out which bushing is best "all around".

Let’s face it, there are no right answers here but merely opinion’s that all matter.

Let’s dig in...

 

In the beginning 

 

Original crankshaft bushings fitted to most vintage British motorcycles dating back to the 1950's (perhaps earlier) where manufactured by a UK based company (still in business) called “Alpha”.

Alpha bushings (called bearings in the UK) where standard equipment on all BSA C15, A7, A10, A50, A65, and other models including early Triumph 350 and 500 unit twins.

Alpha bushings typically featured 2 types of materials for a single bushing:

• Steel / metal housing
• Bronze insert

Alpha bushings where made of a steel property while a bronze liner or insert was pressed in place and held by a dowel or pin.

For many years this form of bushing was a common replacement part until the early 1990's when other manufactures found better solutions and methods for a replacement bushing over the old 2-piece bushings.

New crankshaft bushings on the market allowed builders to experiment with new materials that would last longer and maintain reliability.

The selection

 On the market today we have a few types of bushings produced from multiple manufactures that all have there own types of materials used.

For A50 and A65 models from 1966-1972 we will cover a few different types of crankshaft bushings on the market today

Manufacture	Material
Kibblewhite	Solid Bronze
ESL	Solid Bronze
SRM	Solid Bronze
Alpha	Steel / Bronze lined (OEM)

 

As you can see above, most manufactures (there may be others) are producing bushings out of a solid bronze material.

Some may also be using different priorities such as phosphor, lead or nickel-alloy.

Solid vs metal bushings

 

Ok so the moment you have all been waiting for..

So which is it - solid or a steel backed bushing?

The answer in short is "it depends".

For me, I prefer the solid bronze type of bushings...

Why?

It makes sense (to me) that a bushing for a crank be of one type of material. This would keep cost down and ease of manufacturing.

When the crankcase warms it will expand.. a solid bronze bushing will expand at a similar rate more so than a metal bushing.

If the fit of your metal bushing is not up to spec, as your case warms your bushing "could" spin.

You can apply this theory to valve guides as well..

Although that is my 2cents I believe you should choose a bushing based on your application and what you feel comfortable with.

Both types of bushings will give you long lasting results but one should look at the overall design and function off the bushing.

Keep in mind that not all makes and models have bushings on the market that are supplied in a solid bronze material such as the BSA C15, B40, and others

If you face a situation as such, install an Alpha bushing.

Pro's and con's

Here are some pros and cons revolving around crankshaft bushings.

Solid bushings (Pros)

• Bronze expands at a similar rate as alloy - which means no "spun" bushings assuming the fit is correct
• Bronze bushings come in more practical sizes to suite worn cranks
• Bronze bushings are affordable

Solid bushing (cons)

• none

Steel bushing (bronze insert) (Pro's)

• Easy to obtain
• Easy to install

Steel bushing (bronze insert) (Con's)

• Prone to spin in the case (if the fit is not correct) - steel does not expand at a similar rate as alloy or bronze
• Often times supplied with no dowel / peg to locate the bronze insert

 

Bushings for sale

We stock a few types of bushings for A7, A10, A50, A65 and BSA "unit single" models should you need a replacement.

Here is a quick list of some of our stock...

Make	Model	Year	Bushing Type	Link
BSA	A7 / A10	1950-1963	Solid Bronze (ESL)	Click Here
BSA	A50 / A65	1962-1965	Alpha (Metal W/ Insert)	Click Here
BSA	A50 / A65	1966-1972	Solid Bronze (ESL)	Click Here
BSA	C15 / B40	1958-1966	Alpha (Metal W/ Insert)	Click Here
Triumph	T90 / T100	1957-1967	Solid Bronze (Kibblewhite)	Click Here

 

 Recap

When installing a replacement crankshaft bushing whether a solid or metal backed variety always take into account that your crank journal must be round and your bushing must be properly sized (line reamed, honed, etc).

Always have a competent mechanic perform this line of work 

Thanks for reading 

Thanks for reading today's tech tip post regarding solid bushings and metal / bronze lined bushings 

We hope that you learned something new during this time period.

If you have any questions please drop us a comment in the comment section below

Click here for more tech tip posts 

Rides safe! -CBS