Saturday 29 January 2022

The AJS 33/7 Trophy: Preparing the Gearbox and Clutch

The Sturmey Archer EIV gearbox that I had chosen to put into the bike needed some work before it could be used.

The starting point for the gearbox was an EIV casing – there was nothing inside - which I had acquired some years ago.  Luckily, Norton bought the rights to Sturmey Archer gearboxes in the early 1930s when SA ceased manufacturing gearboxes. I say “luckily” because Norton then had these gearboxes made (by Burman) in large numbers and they also didn’t change much, at least initially, which is useful when it comes to finding parts as they are both plentiful and fit. Up to this time, the positive stop mechanism had been an external “bolt-on” item positioned just above the gearbox casing (the picture below is on a 33/10 - the 500cc version of the bike)

but a redesign by Norton put this inside the casing but with an external linkage

 both of the above pictures courtesy of Bonhams

 ….and the Dolls Head gearbox was born, although the "head" is more pronounced on later versions of the 'box.

The Sturmey Archer/Dolls Head gearbox is well designed and extremely strong; I used a close ratio DH gearbox some years ago in my JAP-engined Levis sprinter and apart from having to face-off the dogs on one of the gears to prevent it jumping out of gear, it never gave a problem. The entire gear cluster and the selector also conveniently fit into the EIV gearbox shell and as I had a spare Dolls Head gearbox (minus the  positive stop mechanism – I had already “borrowed” that for the V-Twin project), I figured that I might as well have the rest of the internals.

All the gears and shafts were in excellent condition and standard ratios.

There is another difference that I am aware of between the EIV and Dolls Head gearboxes, namely the EIV gearbox has a bush on the drive-side end of the layshaft whereas the DH gearbox has a bearing. The bush in my EIV casing was worn and so, rather than replace the bush, I machined the casing to accept a bearing.

There was also quite a bit of wear on the final drive sleeve gear bush, which tends to wear rather than the mainshaft. This was pressed out and a new one made and pressed in:

before final machining to size.

The gearbox was now essentially complete and it was time to turn attention to the clutch ….except that I didn’t have one.

Anyone in the UK that reads my blog will know the meaning of the expression “if it ain’t broke don't fix it”  ….for anyone that is not a native English speaker this translates as “if something works, then don’t change it”. Norton adopted this philosophy in a fairly major way and this is why a clutch from a 1950s Norton Dominator and a final drive sprocket from a Norton Commando (which was introduced in 1967) fit the splines of an early 1930s Sturmey Archer gearbox. I have also just checked the splines on a mainshaft that I replaced on a mid 1920s Sturmey Archer CS gearbox on one of my vintage Nortons and that is also the same.

I needed to have a shock-absorber in the transmission and as this isn’t going to be on the crankshaft or the rear hub, it has to be in the clutch. The Norton shock-absorber clutch is compact and well designed and the first step was to purchase a new “spider” that fits onto the mainshaft (this is the part that drives the shock-absorber) to check wear on the splines.

Absolutely fine; and so the next step was to find a clutch. Various modifications were made to the Norton clutch over the years it was in production – and it was also fitted to AMC bikes (in fact, Sturmey Archer developed a shock-absorber clutch long before the design was acquired by Norton – see the 1931spares manual). To avoid ending up with a collection of incompatible parts it is advisable to acquire a complete clutch rather than trying to make one up from separate bits. A quick search on ebay found a complete Norton Dominator clutch ….unfortunately on a US listing:

Rather than wait until one came up in the UK I bought this one. Unfortunately I had to pay 70 GBP postage plus another 70 GBP VAT/Excise duty/collection fee to get it delivered! Whilst I appreciate the need for taxes to oil the wheels of bureaucracy I really object to paying 50 GBP “Value Added Tax” where the value that was added occurred over 60 years ago, somewhere around 1960, when it was manufactured and fitted to a bike that left the Norton factory and for which Purchase Tax would have been paid at the time. Anyway, rant over ….I have a clutch.

Upon stripping, most of the clutch was in pretty good condition although it was pretty obvious why it would have stopped working!

I don’t know what level of abuse that you would have to give a clutch to break off all 3 vanes of the shock absorber spider in this way.

With lots of new bits - rollers for the bearing, shock-absorber rubbers, bonded lining on the backplate and friction and plain (surflex) plates the clutch was ready for reassembly.

I'm sure there is already plently written on this, but just a quick note on inserting the shock-absorber rubbers: I put in the larger rubbers and then held the spider on the splines of an old mainshaft in the vice. Using a clutch locking tool on the clutch centre splines for the friction plates and, with a length of exhaust pipe for extra leverage, I could easily compress the rubbers

to simply pop in the smaller ones.

The clutch is now as good as new.

The final bits to complete the preparation of the gearbox are a new Norton Commando final drive sprocket for a 5/8’’ x 3/8” chain and LH thread securing nut and washer. The Commando sprocket has a narrower hub than the original and a spacer has therefore been made to avoid the sprocket fouling the gearbox casing and to get the chain alignment correct.

The gearbox and clutch are now ready to be fitted in the yet-to-be-made engine plates.

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