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Triumph clutch on a M21

It is time to get rid of the BSA six spring clutch. I have heard of using a Triumph clutch but never seen the conversion other than on a late swing arm Goldstar. What research I have done, I get different answers. I know I need an adapter to fit the BSA main shaft but am confused. The bike is a 1950 rigid frame M21.

I have read:

* The adapter will only work on a swing arm BSA.

* The adapter will only work on a plunger BSA.

* It will fit but chain alignment will be way off.

* It will fit but only on the earlier bikes with the WD clutch.

* No one makes an adapter for you model gearbox.

* The plunger one will work but you must modify the taper.

So what is the correct answer and who makes the adapter for my bike? Any one done the conversion?

Re: Triumph clutch on a M21

Hi Bruce
Try Kidderminster motorcycles they do a 4 spring Triumph clutch to fit Rigid/Plunger M20/21 with either early or late type gearbox, The adaptor is machined a bit deeper to fit the gearbox swinging arm adaptor will not fit, I fitted one to my 56 M21 AA bike and it is excellent,
Regards John(Tom)

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Re: Triumph clutch on a M21

The original BSA adapter was manufactured to fit only swinging arm gearboxes....As the taper angle of the gearbox mainshaft is the same on all the boxes you have mentioned in your post it will physically fit onto plunger and WD boxes.
However, although the angle of the tapers are the same the diameter for the plunger and WD boxes is different. The effect of this is that the adapter won't go so far onto the shaft of those boxes and results in the clutch sitting further 'outboard'...

This in turn means the clutch and engine sprockets will be out of line...The amount is approx. 1/4". One solution is to fit a suitable spacer behind the engine sprocket carrier that fits over the crank...(if you have the one piece type with a shoulder on the back)..If you have the carrier that is used with a separate spacer a thicker spacer will have to be made that replaces the original...

That will bring the sprockets back into line and there are no issues with clearance inside the outer primary chain case...The additional spacer does put more load onto the crankshaft shock absorber spring though, so the action of the cam must be checked after fitting...If the shock absorber cam won't move or has insufficient movement a shorter spring should be used...

The above only serves to show that the original clutch adapter can be used....The easy solution is to purchase one of the modified adapters which are now available but in the past weren't....Note also that the length of the clutch pushrod may require alteration....Ian

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Re: Triumph clutch on a M21

Kidderminster does not appear to have a web site. I am in California so would prefer to email them. Do they have email?

Re: Triumph clutch on a M21

Kidderminster Motorcycles
00 44 156266679 and e-mail
I have one of there 4 spring clutches on 1951 rigid BSA 31 and it works very well so far.

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