Service Bulletin: Metric Main Bearings

Service Bulletin: Metric Main Bearings

Metric Main Bearing Service Bulletin

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Today's blog post is an original Triumph service bulletin dated 10-31-1972. This service bulletin is in regards to the new Triumph 650 twins metric crankshaft bearing that was installed during mid-1971.

There is a lot of talk about this subject here online but I have never seen an actual Triumph document directly discussing this change. However, I was aware of the change but now we have factory proof. Most folks today have not seen this service bulletin until now. I am very pleased and excited to share it with you all today. Let's get to it...

Metric main bearings

During the 1971 model year the Triumph engineers decided to change the timing side (TS) crankshaft bearing from an imperial (inch) bearing to a metric bearing on the 650 lineup.

This change introduced a new design to the timing side (TS) crankcase, crankshaft, and the main bearing its self. 


Why the switch from an inch bearing to a metric bearing?

The reason for the switch is still unclear to many today. I can only assume the reason for the change was to keep the cost down. Metric bearings have always been known to be "standard" for most applications. Metric bearings are also very easy to obtain and relatively inexpensive compared to inch bearings today.

Although the factory may have been saving funds using a metric bearing it is also counterintuitive because the initial set up to modify the case and crank would be costly. At this time the factory was in a tough financial standing.

One point to also consider is that the late Triumph 500 twins from 1968-1974 actually used the same metric bearing on the timing side exactly like the 1971 and on Triumph 650 twins.

Both applications require a metric bearing in a "C3" condition. It is highly possible that Triumph used the leftover 500 bearings in there 650's.

Although this is all a theory it seems to be practical but we may never know. 

Where does the change begin?


Triumph Engine Number GE27209

 Triumph serial #GE27209 the starting of the metric main bearing

Most of the time Triumph was pretty good at documenting changes and informing there dealers by issuing a service bulletin just like this original one. Sometimes they never gave a direct reason of why changes where made.

The change started on Triumph 650 twins from engine number GE27209. Any 650 Triumph made after GE27209 would no longer be able to accept the typical 1 1/8" bearing.

Below is a chart indicating the models that featured the new metric main bearing.

 Brand Model Capacity Year
Triumph Bonneville (T120 & T120V) 650CC June 1971-1972
Triumph Tiger (TR6 & TR6V) 650CC June 1971-1972
Triumph Trophy (TR6C & TR6CV) 650CC June 1971-1972

Do not interchange parts!

It is vital to recognize which crankcase, crankshaft and bearing that you have. You would be surprised of how many incidences that we've had when customers bought an inch bearing for there engine when there application required a metric bearing.

When purchasing parts it is vital to check your crankshaft and crankcase. Also, when you're looking for a second hand crankshaft or timing side crankcase it is also important to ensure that each part will except the same bearing whether it be the inch bearing or a metric bearing.

So if you think you scored a nice crankshaft off of eBay you might want to double check your parts because it's very possible that you're crankshaft, bearing or case might not be compatible with anything that you have.


Inch Vs Metric


70-1591 Bearing Vs 70-3835A 70-1591 inch bearing compared to a 70-3835 metric bearing

It is rumored that the inch bearing is stronger than the metric bearing. One of the downfalls of the metric bearing is that the 8 individual ball bearings are smaller than that of the inch bearing.

We have also heard and read that there where many ball bearing failures in the T140 and TR7 models as they used the same metric bearing for a certain extent.


 Below is a chart indication each bearing size and ball bearing count.

Bearing Type Dimensions Ball Count
Metric bearing 72mm X 30mm X 19mm 8
Imperial bearing 1 1/8" X 2 1/2" X 5/8" 8


Part numbers 

Below are all the part numbers which include the crankcase, crankshaft and bearing. We also show the "early" and "late" part numbers used when fitted with an inch bearing and metric bearing.


Part Type Part Number
Metric Bearing 70-3835 or E3835
Inch Bearing 70-1591 or E1591
Crankshaft (early inch bearing type) 70-4643 / E4643
Crankshaft (late metric bearing type) 71-3097 / E13097
TS Crankcase (early inch bearing type) 71-2274 / E12274
TS Crankcase (late metric bearing type) 71-3098 / E13098



So there you have it! When replacing parts it is always vital to check what parts you have and parts you need for your application. So what do you all think? Do you think this "change" was a smart move from Triumph to install a smaller metric bearing?

Let us know what you think by commenting in the section below. Don't forget to check our some of our recent blog posts just like this one by clicking here. Ride Safe...


  • Sluggo

    just a guess,,, but today we have ISO standards and there is newer versions, I would have to ask my wife what they new standards are but the idea is globally products are all made to known standards. For example Yamaha, Suzuki and others at one time had all their own unique fasteners. So,, starting in the late 1960s and into the 1970s there was early efforts at standardization and eventually also the metric system which never fully took hold here in the US.
    So, keep in mind the engineering depts at BSA-Triumph was in massive change and flux and new corps of engineers many of which from Aerospace were brought in and the whole “Umberslade hall” design and development dept took over. (Also known as “Slumberglade”) Probably and likely these bright bulbs thought updating to pending world wide globalization stds were a good idea. In practice this seems logical and prudent,, but in practice did not work out well.
    Hell,, I have an old Chevy with a weird mix of metric and std US fasteners on it.
    Used to fun back in the 70s and 1980s to take a whitworth bolt into a small town hardware store and tell them it fell off some piece of farm equip and watch them scratch their heads…

  • Classic British Spares

    @David – That is correct! Many Triumph models used metric wheel bearings and many used imperial. Thanks for reading and commenting today!

  • David Taylor

    Triumph also used metric bearings in the front wheels, friends of mine knew when they had metric bearings in there bikes as they wore out at around 10,000 miles unlike the good old imperial that kept soldering on.
    Thank goodness I rode a good old imperial BSA rather than a cheapskate Triumph.

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