After months of trying to source an original clutch it became obvious that there just wasn’t one available. A friend of mine in the Club said no worries we will make one. (I’m not naming people because I might forget someone who helped me). I had one component of the clutch and that was the splined ‘clutch driving member’ that fits the driven shaft of the gearbox. The clutch driving member also has a large diameter track to take the bearing on which the basket runs. I was fortunate to have this component as manufacturing it would entail machining splines and case hardening the bearing track. The basket that holds the clutch friction discs and clutch plates was spun. A flat steel disc of suitable thickness is retained in a lathe and as it turns the operator applies a tool with a long handle and bends the metal as it turns until the metal is shaped as in the picture. Slots are then cut to take the friction plate lugs. The first attempt using the spun basket didn’t work too well. I welded a band around the basket to stop it opening up as pressure was applied during the operation of the clutch but the addition of this band caused some distortion and balance problems. The fix was to machine a basket out of a piece of large diameter aluminium bar. Another friend machined the basket and milled the slots to take the clutch plates. New AJS parts are used for the plates, the clutch springs are modern Triumph and rollers are used in the bearing.
The clutch is driven from the engine via a chain wheel attached to the basket. The number of teeth that can be accommodated on the clutch chain wheel depends, to some degree, on the overall diameter of the clutch which itself is governed by the diameter of the clutch plates. The clutch and chain wheel and chain have to fit into the primary chain cover which is designed to accommodate fore and aft movement of the gearbox to allow tensioning of the primary chain. The unit had to be made to fit or else the primary chain case cover would need to be modified. My main consideration, however, was to keep the final ratio the same as the original bike. The ratio is all about how many times the engine crank shaft turns over for one revolution of the back wheel. If the ratio is considerably different from the factory spec. the engine will either be revving fast or labouring in a particular gear for a given road speed. Apart from the design of the gearbox, the ratio depends on the number of teeth on the engine, clutch, gearbox and rear wheel sprockets. In my case the gearbox and rear wheel sprocket were factory original and the clutch sprocket was fixed by the dimensions of the homemade clutch so the only sprocket I could change to achieve the original ratio was the engine sprocket. The final ratio is calculated by dividing the product of the number of clutch sprocket teeth and rear wheel sprocket teeth by the product the number of engine sprocket teeth and the number of gearbox sprocket teeth.
The original ratio for the bike in top gear was 5.06:1. That is the crankshaft turns 5.06 times for one turn of the rear wheel in top gear. I had the clutch with 36 teeth and the rear sprocket with 38 teeth. Multiplying these together we get 1368. The gearbox sprocket had 15 teeth so I chose 18 teeth for the engine sprocket. Multiplied together, 15 and 18 equals 270. Divide 1368 by 270 and we get 5.06 which is the ratio required. I bought a blank sprocket with 18 teeth, machined the original engine 15 tooth sprocket off the boss and welded the 18 tooth blank in place. When everything was assembled it all fitted OK in the primary chain case cover. The slotted holes in the frame that take the gearbox retaining bolts were long enough to allow proper adjustment of the primary chain which meant that the rear wheel chain adjustment available was still OK. It all looks good but the question yet to be answered is does it work?
After the clutch was finished my attention turned to the handle bar fittings and cables. I emailed UK seeking a period clutch and brake handle. I should have said that I want a period clutch handle and brake handle because the recipient read it as I wanted a clutch and a period brake handle. He could supply both. After making one I was now offered the genuine article. I had to buy it of course and it came in excellent condition with new friction plates. To fit it to the bike I would have to change the engine sprocket again so I’ll see how the home made one goes. Handle bars, fittings and control cables can be a bit fiddly but that’s story for another day.