Vintage Workshop
Services for Brough Superior motorcycles and their contemporaries

Sturmey Archer gearbox page last update: 07/2022

After looking at all the bits I have accumulated over the years I came to the following conclusions: - Grease'n oil lubrication is the accepted compromise to keep some lubricant inside the gearbox, but it is not really a good thing. Have a look at these three main shafts: All of them have scoring marks from running inadequately lubricated in the sleeve gear. Measuring them up showed me that they were significantly worn, especially at the outer end.
- The sleeve gears I had showed a similar trend, with their bore being significantly larger at the outer end.
- Altogether the running clearance in the sleeve gear, which I am told was originally 5 thou had thus increased to 10 or 15 thou. This is not only a clearance where the whole setup can no longer work as a hydrodynamic bearing, but also the mainshaft can no longer keep the sleeve gear well enough aligned for the ball or roller bearing holding the latter to work for any significant mileage.
- besides that, the clutch splines on most of these shafts are a bit worn, which means the clutch centre will not seat properly, and will continue to wear the splines out.

I made some enquiries about hard chroming or metal spraying the portion of the shaft that goes through the sleeve gear, but gave that up because of the worn splines and some other issues.
So I sat down and made a drawing.
Which made me realise that it was not impossible to make such a shaft by my own means, but a demanding task...

So I obtained a metre of 18CrNiMo7-6 case hardening steel and set to work. First job was to drill the bore for the clutch actuating rod. You need to drill from two sides, but only after VERY carefully centre boring the respective end. If you start drilling only slightly out of centre, your drill will be way out when it meets the bore from the other end. And you need to drill quite gently, removing the drill every few millimetres to clear the swarf - you don't want a broken drill stuck in your piece of work... Actually, this is why I did this tedious job up front, so only a piece of material would be lost in case...

Rough turning the job was nothing special, save for the fact that the material produced long hot and sharp swarf which - in the best case as seen here - wrapped up around the work. Sometimes it would curl up and point in my direction...

This is how it looked then.

I then encased the bit in a sand-filled piece of steel tube and annealed for two hours it in my wife's pottery kiln at 600 to relieve any potential stress in the material in the hope of thus minimising the distortion in the hardening process.

A measuring session before and after showed no measurable difference, so I could have saved this effort...

Before having a go at milling the various splines I tried things out on a dummy piece.
here I am roughing out the splines for the sliding gear...

... and here I am cutting their sides.

Now I could round out the base circle between the splines in a series of cuts...

... after which the gear slid nicely on.

Similar procedure in cutting the splines for the fixed gears...

Only difference here is that you want the width of the splines correct to less than 1 thou, especially for the clutch. You'd really want to have a nice snug sliding fit without any shake!

I made a new layshaft as well, especially because the sliding gear and the kickstart gear were a pretty loose fit even on a NOS layshaft I had. Without boring you with any further small steps - here are the two shafts almost finished.

As mentioned before, I found all the gears having rather large running cearances on their respective part of the shafts - typically some 10 thou. I don't know if this is due to the wear and tear of the decades or if it was like this from new. If the latter, then it is probably due to the fact that you could buy spares that needed to fit, taking all the manufacturing tolerances over the years into account. Or did Messrs Sturmey and Archer consider this necessary for the grease lubrocation of those days? Be that how it may, if precisely manufactured and running in oil, one or even a half thou should be just right.
So I made all the shafts to give a nice fit, having cleaned up the inside of the gears before.
Here I am just lapping the bore of the kickstart wheel to bring it back to round and cylindrical. If you happen to have a dismantled LS gearbox on your bench, and you have a good 3-point inside micrometer, measure the diameter at both ends and in the middle of the gear and I bet you will find it conical and trumpet shaped at the same time!

... to be continued...


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