From day one that photo appeared to show the clutch drum sitting too far out on the main shaft.
I usually do my trial assemblies without the chaincase on as it makes proper alignment much esier.
The two sprockets must be as close to perfectly in alignment as possible or you will grind the teeth off the drum sprocket in no time flat

BEcause BSA made all of their gearboxes apart from the plunger bolt on one all of the mainshafts are very similar and will usually fit into the boxes
The taper on all of them is the same but the clutch centers are different on order to move the clutch inboard or outboard .
Please do what Bruce suggested and also measure the distance from the back of the clutch basket to the chaincase .
Let us know what you find
It could also be the sleeve gear bushprotruding too far out of the end of the gear preventing the clutch centre locking on the taper .

Can I briefly put in a query about my own case?
I have 2 cush drive assemblies, one each of the types mentioned here.
Making the measurements as Barry has done in the above photo, the clutch sprocket is 1/16 inch proud. I'm wondering whether to take this up with a couple of standard spacers, or would it be better to get single spacers made to measure?
The cush drive is certainly positioned correctly, when I previously fitted the clutch it seemed right.

['Because BSA made all of their gearboxes apart from the plunger bolt on one all of the mainshafts are very similar and will usually fit into the boxes...']

Not correct I'm afraid...

The plunger A7/A10 bolt on box, the 4 speed C11G gearbox and the swinging arm gearbox are all of essentially the same basic (BSA) design with detail differences in mountings, gearsets, gearchange mechanisms etc...However, mainshafts from all three boxes are interchangeable...The swinging arm gearbox mainshaft has a taper and either the BSA six spring clutch or the Triumph clutch can be fitted with the appropriate adapter...The Plunger A7/A10 box has a spline so will accept the duplex chain plunger clutch...This shaft is also frequently used in the s.arm box as the spline is the same as a Norton and so there is the option of using the Norton clutch...(for Goldies etc. usually)
The C11G 4 speed box has its own shaft and its own clutch etc. and these aren't generally used in other models....
None of the above mainshafts shafts will fit a heavyweight box of the type fitted to a WDM20 , the post war models rigid/ plunger B Series and M Series or the 46/47 B31 ...
However, although the WD box and the later heavyweight types are visually different the mainshafts in these are identical and interchangeable as are nearly all internal parts....
Whilst the taper angle is the same in these boxes as swinging arm types/variants the taper diameter is different...Thus the problem when trying to fit a standard BSA clutch adapter for a Triumph clutch to a heavyweight box....
B31 gearboxes from 1946/7 have a shorter mainshaft of a different design and these are also not interchangheable with heavyweight boxes or the later s.arm boxes....The mainshaft is common to the B29 and WB30 box from which the lightweight B31 box was developed...So, due to the shorter mainshaft those boxes had a different version of the six spring clutch adapter with the taper moved within the adapter to align the clutch with the B29/B30 and B31 engines, which were wider than the pre war B Series engines that used the box originally...For that reason the six spring adapter from later swinging arm models and the swinging arm adapters for Triumph clutches are not suitable for these boxes...I had to make substantial alterations to an M20 clutch adapter and the clutch to fit a Triumph clutch to my WB30....Ian

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

Thanks everyone for helpful suggestions. Perhaps I used the wrong terminology when describing the clutch sleeve “bottoming” out on the shaft. I use a large socket and hammer to drive clutch center onto shaft so I know it can’t go on further and I have to use a puller to get the sleeve off the shaft. Distance from back of clutch sprocket to chaincase is 3/4” measured as shown. Thanks again for assistance.

A friend made up a shim for me and sprockets are now in alignment. Now just waiting for correct Cush spring and I can finish assembling the chaincase. Thanks everyone for the guidance and advice. This site is such a great resource.
Regards,
Barry

Hi

Just a few things to check down the road:
1. When you put the chaincase cover on make sure that nothing is in contact with that. I had an over deep pattern shock absorber nut which ground a nice circle in my chaincase. You might have a similar issue if the shock absorber is too far out. Equally it could all be ok if the underlying issue was the shock absorber. Just with checking
2. Is the spacer hardened? Trevor made an excellent point somewhere in a similar post, which is to use an old bearing inner cut and ground to the required width, that’s just easier than hardening and tempering mild steel. I think the spacer for the shock absorber should be hardened, but Ian would be better to advise on that.

email (option): dickie.bobbie@hotmail.co.uk

In an ideal world I'd recommend the spacer was hardened as they do wear, particularly so if the engine shock absorber isn't working correctly...Mark Cook makes the 'std.' thickness spacer to that specification...
If hardening facilities aren't available a material such as EN19 would make for a reasonably durable component..In other words avoid basic mild steels such as EN1A....Ian

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

Thanks again for all helpful replies. I definitely will check chain case clearance when I get the cush spring installed. Not sure if the shim was hardened. He used some shim stock that he had. He’s out of town this weekend but I’ll check with him when he returns. Thanks again.
Barry

As it seems there are three different thicknesses of these hubs, it might be more cost effective to make only one size and two spacers?

Mark

email (option): pes.sales@btconnect.com

Definately more cost effective considering the cost of cutting the splines and grinding the inner race region
However unless you want to ship them asa set , which wastes materials it will need to be well documented on the PES web page so purchasers can order the correct one .
As a person educated in the engineering sciences and a true conservationalist I hate wasting materials .