Friday 6 May 2022

The AJS 33/7 Trophy Model Restoration: The Primary Drive


Although the gearbox was now in place and the Norton clutch had been rebuilt the primary drive was not yet complete. The engine sprocket that had come with the bike appeared to have been made from part of an original crankshaft shock-absorber with a sprocket welded on ….seen below.

This would probably have worked OK but there was a problem with the positioning of the clutch. It’s location on the gearbox mainshaft, seen below,

was too far out and this would have resulted in the engine sprocket also being located too far out on the crankshaft at around the end of the taper section. This would not be a good design feature and I therefore decided to move the clutch inboard by 5/16”.

Moving the clutch along the mainshaft is not a 5 minute exercise! Essentially, the splined section needs to be moved inboard and this means extending the splines along the shaft and reducing the diameter of the mainshaft to the thread diameter for a similar length (right hand side of the picture below).

and after pressing off the gear on the opposite end of the mainshaft

I have made a gearbox mainshaft in the past for the Sturmey Archer CS gearbox on my 1929 M18 Norton and although it is quite a bit of work, it is relative straightforward. The picture below shows a worn original CS gearbox mainshaft (above) with the new one (below) that I made. 

Although I don’t have the bike any longer, I put a few thousand miles on it, including a couple of visits to the Isle of Man for the Manx GP and a rally around Italy and Sardinia.

However, the existing mainshaft that I needed to modify here is through-hardened steel and it is simply not possible to set it up in the lathe and milling machine and machine it. It would be straightforward to reduce the diameter by grinding but I do not know how to extend the splines in hardened steel using traditional machine tools, at least, not the ones that I have. And so, back to my faithful spark-eroder who did both jobs. Copper electrodes were made to reduce the diameter

and extend the splines.

After making a new and extended nut to support the clutch centre, the complete clutch could now be assembled.

The last step was to make a new engine sprocket. The engine sprocket that came with bike, seen as the first picture in this blog post, was not a pretty sight. However, as the taper was a reasonably good fit on the crankshaft, the first step was to use this to machine a male taper to fit.

As I’ve mentioned in previousposts machining a sprocket with an internal taper has to be done with great care to locate the teeth in the correct axial position. As with previous engine sprockets, this one started out as a triplex sprocket with 3 rows of teeth, 2 rows of which were removed

before machining the taper. The male taper is used to check the accuracy of the female and to ensure correct positioning.  Finally, the keyway was put in using spark erosion.

The primary drive is now complete with pretty well a new clutch, engine sprocket and chain.


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