Tech Tip: Sintered vs Bronze Fork Bushings (1964-74)
There has been much controversial talk covered about which camshaft bushing material, valve guide material and even down to clutch sleeve thrust washer material should be used in an engine or application.
Over the past 5 or 6 decades just about every technical topic or question has been addressed in books, manuals and here online through popular vintage British motorcycle forums.
Triumph top fork bushings - solid & sintered bronze
Today I wanted to talk and discuss with you something that doesn’t come up often or not at all? This topic is in reference to fork bushings; sintered vs solid bronze bushings and why each bushing material will give you different results.
I would also like to take the time to help you choose and consider which fork bushing will better suite your needs and application for Triumph 650 models from 1964-1970 and Triumph 350 / 500 models from 1964-1974.
So before you purchase your fork bushings please read the following...
Why this topic?
1970 Triumph front forks ready to be fitted onto Kyle's race bike
I was motivated to write this tech tip post when I was putting together my front forks for my Triumph 3TA road racer. I had to replace my fork bushings and was stumped on figuring out which bushings I should install.
After taking the time and figuring out what I needed I then quickly found that I’m not the only that needs help getting through a topic such as this one.
Fork tubes for Kyle's race bike... notice the (2) small damper holes on each tube filled in?
The forks I acquired where purchased off of eBay and where taken off of a 1970 Triumph T120R. I wanted the 1970 forks because I like the bottom studs (instead of bolts) and I like the anchor lug on the timing side fork leg.
Upon disassembly I noticed the top and bottom fork bushings where worn. I grabbed my mic and Triumph workshop manual and started to compare measurements. Bottom pivoting bush was about .008” worn and the top bush inner diameter was about .010” worn which is ALOT in my opinion.
Looking at the forks as a whole and the bushings I removed and all other parts it would have been safe to say that the front end was "untouched". In other words I will assume it has never been apart.
Selecting replacement bushings
As you may be aware when replacing fork bushings you will find that some suppliers are offering bushings that are solid bronze while others are sintered bronze or both (including myself).
Original Triumph fork bushings where sintered bronze while BSA fork bushings (pre-1969) where solid bronze (for the most part). There is a reason why Triumph and BSA selected which type of bushing materials to use in there fork assembly's.
Differences in materials
I’ll go over quickly what each material means and there property’s because that will dictate what application a bushing can be used in.
Sintered bronze (aka oil lite)
Sintered bronze is a type of bushing that is also referred to as oil impregnated bearings or self lubricating bearings.
Powdered bronze is poured into a special die and then subjected to about 30 to 50 tons of pressure to form the bushing into the shape that it is desired. Very similar to forging
After the bushing is manufactured hot oil is then forced into the pours. When a centered bronze bushing is in service, heat and friction will allow the oil impregnated to properly lubricate the inside of the rotating or moving shaft.
Solid bronze materials are typically made on a CNC machine or a lathe. A much different manufacturing process from centered bronze.
Solid bronze bushings are stronger, can hold a tighter tolerance and are typically more expensive then centered bushings. Also not to forget, solid bronze bushings (depending on properties) are typically prone to premature wear.
Which bushing do I need?
Ok the tricky part...
Without starting too much controversy both solid and sintered fork bushing will work just fine for the average rider.
Based on our fork applications I believe that sintered bushings are best suited.
In the modern world everyone wants better quality parts, which is superb but one must not forget why the old British selected a certain type of bushing material for a given application.
The way to go (in my opinion)
Sintered bronze (the way to go) as stated above retains and holds oil like a sponge. It’s porous property’s and soft material makes it much suitable and more forgiving when used in forks. This also prevents premature wear on the fork tube and leg giving a longer service life.
Solid bronze bushings are awesome and really don’t wear out due to there properties but often times we have seen that the solid bushings can wear out fork tubes and fork legs out quicker than sintered bushings.
Solid material bushings do not "absorb" oil like sintered bushings making them more prone to wear.
It’s a trade off
Although I’m an advocate for both types of bushings being used in fork assemblies, my personal preference is sintered bushings because they are more forgiving, affordable, and can lubricate and extend the life of your front forks and fork legs.
Select your fork bushings
If you are putting together your forks right now that happen to fall in the Triumph 1964-1974 range then now would be a good time to select what type of ford bushing you desire. As stated above, we do stock two different types of bushings that that can be used for your application.
Feel free to take a look at our chart below and click on any of the links to purchase your for bushing set today.
Triumph Fork Bushings (350 / 500 1964-74) & (650 1963-70)
|Solid Bronze Fork Bushings||Click Here For Link|
|Sintered Bronze Fork Bushings||Click Here For Link|
Thanks for reading
Kyle and Malcolm here, we just wanted to say thank you for taking the the time to read today's tech tip post!
If you have any questions or if you wold like to leave a comment, please use the dedicated commenting section below.
Thanks for reading and ride safe!