If I had dismantled my WM20 myself, I would have made copious notes and taken lots of photo's and video, however, it came to me in a number of carboard boxes, leaving me to work out, not only, what all the rusty bits and pieces were for, but just as important, what was missing.

I'm pleased to say that I've sorted most things out, but on coming to assemble the primary drive, I have a problem of not knowing if something is missing or if things are as they are meant to be.

With the clutch basket pushed as far onto the shaft as it will go, I find that, if I put a steel rule accross the face of the chain ring, there is a gap of about 4mm between the ruler and the face of the sprocket on the cush drive. Is this normal or am I short of a spacer on the cush drive?

At first I thought that maybe the woodruf key or the cork seal was preventing the clutch from seating properly onto the shaft, but with them removed there is no change in the clutch basket's position.

Hope that all makes sense!
Scorp.

email (option): wm20atscorpionvideo.net

you should have a thrust washer behind engine sprocket, check that first if not then check clutch is on fully...don't run with 4mm out of kilter, chain will wear quickly or even snap with unpleasant results!

email (option): barryrapley@sky.com

Thanks for your reply.
Is this where the thrust washer should go...

...if so, I haven't got one. Is this just a washer that I could make, and if so, can anyone tell me what are it's dimensions and what it needs to be made of?
Scorp.

email (option): wm20atscorpionvideo.net

Most engines don't have a thrust washer but after owning a few m20s I have found a few, the thrust face the sprocket sits against can wear, so pehaps during rebuilds thrusts were added?
And yes that is the correct position for thrust washer.

p.s you could try a different cush drive assy, without any wear?

pps. Russel motors can supply engine shaft shim and distance collar.

email (option): barryrapley@sky.com

There seems to be some confusion here...

There are two types of the component that the engine sprocket fits onto..(crankshaft sleeve)

The first has a shoulder machined onto the rear face (as shown in the photo)..This shoulder fits into the recess in the side of the crankcase and rests against the outer main bearing oil retaining washer.

The second type does not have the shoulder machined onto the rear face.
In this case there is a separate part, basically a collar of the same dimensions, which fits into the recess before the crankshaft sleeve is fitted.

In either case the final position of the sprocket relative to the clutch sprocket ends up the same...They should be in alignment.

However, to cater for production tolerances or slight wear there are shims that can be fitted to correct the alignment...NOTE: shims...not thrust washers.

These shims are only intended to correct misalignment running to .010" - .020" (thou) or so and they are fitted either between the spacer and the crankshaft sleeve in the case of the 'two piece' set up or, when the one piece sleeve is used, against the bearing oil retaining washer before the crankshaft sleeve is fitted...As such, they are clamped in place when the assembly is tightened up...

They are NOT intended to be fitted directly behind the engine sprocket...

If you have a misalignment of as much as 4mm you should look elsewhere initially for the problem.
If for example the gearbox is not fully tightened up in the gearbox plates it may not be sitting 'square' in relation to the axis of the bike...A slight misalignment here will be magnified over the distance between the clutch and the engine....Ian

email (option): ian@wright52.plus.com

I just turn my distance collars up on the lathe to suit.

Hi Douglas...Yes I've done the same thing myself when using the 'two piece' set up...

However...4mm (3/16") is a large discrepancy when standard parts are being used and it would be wise to establish where that sort of inaccuracy originates...Ian

email (option): ian@wright52.plus.com

I have checked the position and tightness of the gearbox mounting, all of which seem to be correct. As the gearbox mounting plates and the spacers on the rear gearbox fixing bolt dictate the position of the box, I can't see how I can make any further adjustments. I will take some photographs which may be of use in identifying the problem.
Things were going so well!!!
Scorp.

email (option): wm20atscorpionvideo.net

Do you have the gearbox plate spacers fitted correctly?...The outer ones are different lengths. Also have you remembered to fit the rear brake torque arm to the same stud next to the spacer on the clutch side?....Ian

email (option): ian@wright52.plus.com

Are the front and rear yoke plates in line...? Due to distortions both of the front and rear frame the result was 4mm at the engine sprocket...

email (option): viaconsu [at] planet'dot'nl

I've had clutch centres with a variation in the taper, I also read somewhere that others had discovered the same when comparing different ones. There was a 3mm difference between two centres I had when offered onto the same gearbox.

There are differences in the clutch sleeves fitted to different models....A,B and M Series models all have the same mainshaft taper angle but the diameters vary depending on the clutch and/or gearbox type used for different models....

So, for example, if you fit the clutch sleeve intended for a swinging arm gearbox and Triumph type clutch to an M20 you will have a misalignment, as the diameter of the S. Arm clutch sleeve taper is smaller, thus moving the clutch 'outwards' in relation to the engine sprocket..

If you have a 'six spring' BSA clutch fitted there are a number of different clutch sleeves for different applications that can cause the same effect.(to such a degree that some of them cannot be fitted at all)..

However...If you have a WD M20 with a standard single spring clutch fitted there would be no major discrepancy in alignment between one clutch and the next...

I have never come across any tolerance variations of that magnitude between any representative selection of a particular BSA part...Ian

email (option): ian@wright52.plus.com

Yes I don't believe either that BSA tolerances were that wide for genuine wm20 parts, my particular experience was the various 6 spring clutches as you say... but I did read on a forum sometime last year a guy stating that a batch of aftermarket BSA clutch centres got produced with incorrect taper positions & were sold to unsuspecting owners, it may well not have been the WM20 type & it's even possible they were simply genuine ones marked up for the wrong machines. but .......if there's miss alignment & all else seems correct it's certainly something I would check so that could be discounted.

The incorrectly machined batch were swinging arm BSA clutch sleeves made to fit a Triumph clutch to a BSA (as BSA did themselves)...

I had to sort one out for a friend after he'd bought the right part from a local dealer but it didn't seem to fit correctly....Ian

email (option): ian@wright52.plus.com

Many thanks for all the comments, on the basis of which, I will be spending the weekend trying to work out what's wrong, and will report back.
Scorp.

email (option): wm20atscorpionvideo.net