Vintage AJS: Rebuilding KTT 55s Gearbox and Gearbox #1

Having just completed the Rebuild of KTT 305s Gearbox, the laser welding repairs to the gearbox shell of KTT 55 and the end cover of gearbox #1 from an early 3 speed gearbox for one of the flat-tankers came back from EMP tooling.

The damage to the end cover of gearbox #1 was the more significant. You may recall from a previous blog post that the end of the bearing housing had been detached – see picture below which shows the damaged cover, on the right, compared to a good one - by somebody trying to remove the mainshaft through the bearing in the past.

I had cleaned up the surface of the end of the bearing housing, made an aluminium ring and then asked EMP to weld it to the casing. This was the result.

Now, I am not a time-served welder – I stick bits of metal together to the best of my ability using TIG or gas, but I imagine that anyone trained in the skills of welding would concur that this is a work of art.

To digress for one moment…. many years ago, when our 2 daughters were small children, one of the bedtime stories I used to read them was The Tailor of Gloucester by Beatrix Potter. It is a lovely story about how an old man, the tailor, is unable to finish making a waistcoat for the Mayor of Gloucester because he gets sick. Jumping to the punch line, mice, that live in the walls of the old house, sew the waistcoat for him and the stitching is so fine and exquisite that the tailor becomes famous. Nobody knows that the mice did the sewing – the last line of the book is: The stitches of those button-holes were so small—so small—they looked as if they had been made by little mice! If you want to watch it on Youtube you can see it here – the relevant bit is at the end.

So, what is the point of this little ditty? When I see the laser welding in the above picture, I think of those little mice sewing the button holes to produce a piece of work as exquisite as this.

Anyway, back to rebuilding gearbox #1. Measurements were taken for the layshaft end-float – the end cover-to-bearing distance

(the block of metal under the measuring face of the height gauge is a 1” slip gauge) and the shoulder surface for the bearing on the layshaft to the mating surface

and, including an allowance of 0.035” for the gasket, gives an end float of:

(0.419” + 0.035”) – 0.46” = - 0.006”

With a 0.019” (0.5mm) shim behind the bearing,

the end-float becomes 0.013”, which is quite acceptable.

After checking the mesh of the sleeve gear and layshaft gear, which were fine, the gearbox was assembled and checked that all gears select – all OK.

Out of the 3 gearboxes that I have so far refurbished, this is the only one that has a kickstart – neither KTT 55 nor KTT 305 have the facility for fitting a kickstart. One important point to check before putting the layshaft into the gearbox for final assembly is that the flat on the end of the kickstart shaft is in the correct (on the upper side) position for the kickstart cotter pin when the kickstart lever is fitted

by positioning the B-14/3 cam in the correct position.

The cam can be put in either of two positions 180⁰ apart and it is the location of the stop on the outer part of the cam coming up against the stop in the gearbox shell that determines the “rest” position of the kickstart. If it is the wrong way round then the flat on the shaft for the cotter will be at the bottom which will position the kickstart pointing vertically downwards instead of up. Not very convenient for starting the bike!

A new C30/2 clutch thrust pin was made from silver steel and heat treated and a new roller type thrust bearing fitted (see previous blog post for details).

A pack of 10x ½” UNF (20 TPI) nuts was ordered and 5 of these were turned into castellated nuts (part BK-50) for the sprocket end of the mainshaft

using the rotary indexer and a 2.5mm end mill – the picture below shows these together with my one-and-only good original and 4 unmachined nuts (these will be reduced in width and used on the other end of the mainshaft – one has already been used on KTT 305s gearbox).

Finally, I was missing an oil filler plug for this gearbox. These are available for later models, which have a different oil filler, from both Grove Classics and Velo Spares Ltd however the larger plugs for the early gearboxes are not listed for sale.

The only option was to make one from a 1” diameter piece of brass by copying one from one of the other gearboxes.

The thread on the original is 13/16” diameter and 13 TPI and I cannot find any standard British, American or metric thread that comes anywhere close to this (….just out of curiosity, please message me if you know differently). Not that it really matters as my lathe can screw-cut a 13 TPI thread without a problem although the thread is so deep that a standard threading tool would not have cut deep enough and I had to use a tipped tool,

DCMT 07 02 04, normally used for regular machining, to get the required depth of cut. It’s not perfect but as close as I could get without grinding a one-off HSS tool.

The rebuild of gearbox #1 is now complete.

The masking tape is to stop various loose bits, such as the thrust bearing, falling off and scattering rollers over the workshop floor. Having misplaced a roller from a big-end assembly in the past (see end of this blog post) I am now especially careful.

KTT 55s gearbox didn’t need too much work. The damage to the shell of KTT 55s gearbox, shown below,

was more untidy rather than structural where somebody in the past had knocked off the corner at the boss of the casting where the 5/16” BSW threaded adjuster enters.

This was repaired.

After checking the mesh of the sleeve gear and layshaft gears

it was found necessary to add 2x 0.019” + 0.011” shims behind the sleeve gear

to achieve alignment.

Rebuild of the first 3 gearboxes is now complete.

In summary, all parts have been checked and any structural damage has been fixed as best as possible, all of the bearings have been replaced (with the exception of 2x sleeve gear bearings that were OK), layshaft end-float and sleeve gear/layshaft gear mesh engagement has been checked and corrected where necessary, new thrust bearings have been fitted, new thrust pins have been made and fitted and a set of new CR gears was fitted and a new kickstart shaft made for KTT 305s gearbox.

There is still a bit of outstanding work to be done – I need a BK-12/2 kickstart for the 5/8” diameter shaft of gearbox #1 (I have 3 later ones with 3/4” diameter – which are of no use because, whilst I can sleeve the main hole for the kickstart shaft, the centre for the cotter pin is too far away from the centre hole and that is not so easy to change) and a couple of B-29 ball joints – these are actually quite complicated little parts – see picture below of all the constituent bits of a deeply corroded one

….but I’m sure something will turn up.

And finally… Apart from the usual spanners, screwdrivers etc I have found that 2 tools are particularly useful in disassembling and assembling these gearboxes.

The first is a bearing puller – this is my set