Wednesday 25 May 2022

The AJS 33/7 Trophy Model Restoration: The Gearchange Positive Stop

As with the V-Twin, the positive stop for the gearchange is mounted externally …and I didn’t have one.  The positive stop that was originally fitted to these early OHC AJSs was, I believe, enclosed in a sheet metal casing. I never seen one “in the flesh” and the best that I could find is a picture of an apparently unrestored 34/10 model that I have screenshot from a Bonhams auction website: 

Picture courtesy of Bonhams

I did not have another spare complete positive stop mechanism but rather a collection of bits that I had acquired somewhere:

I asked around my buddies if anyone had a spare positive stop and Ron Langston came up with a gearbox front cover incorporating a complete positive stop mechanism:

I have no idea of the manufacturer of this gearbox/positive stop or to what bike it would have been fitted. If anyone that reads my blog has any idea, please let me know. I decided not to use it as it was for a 3-speed gearbox.

Ron also had a collection of various dolls-head positive stop bits and pieces and, together with the bits that I had, I was able to assemble a complete positive stop and in good condition. I had to make the spring-loaded indent that goes into the rear of the mechanism but that was a small price to pay.

And so, the starting point was a complete mechanism plus a lump of round steel to make the casing.

After a couple of days of machining the main body of the casing was made (I had to reverse-engineer the one that I had already made for the V-Twin):

and with the addition of a bracket:

The complete positive stop could now be attached to the engine plates:

and with a front cover and a new linkage

I could now select all 4 gears without a problem.

The remaining tasks are to fit the gear indicator and sort out the gear change lever but that can wait until I have the footrests in place.


UPDATE ON 13/07/2023

When road testing the bike I found that the gear change was very sloppy. I fitted a new return spring (obtained from RGM Norton, who I thoroughly recommend) and the gearchange is transformed.

The picture below shows the old and new springs and the reason for the poor gearchange is immediately apparent.

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