Tuesday 3 November 2020

The Chassis

When I started this V-Twin project I had absolutely no idea what chassis – frame, forks, wheels etc.. the engine would go into or what gearbox I would use.

I had already made the patterns and crankcase castings when an advertisement appeared on a well-known UK website, CarandClassic, for a 1931 AJS SB8. The SB8 is one of the last motorcycles that AJS produced as an independent motorcycle manufacturer in Wolverhampton before selling the business to the Collier Brothers and, by this time, the way in which the engine and gearbox were mounted had evolved from the earlier “flat tank” models by having a very significant amount of space between the front and rear frame down-tubes, clearly illustrated in the 1931 AJS brochure picture.

The bike being advertised wasn’t in quite the same state as the brochure picture…

and cost me considerably more than £47 but all the main chassis parts were there. It is an AJS of the correct era and, most importantly, it had registration documents ….a real bonus. A deal was done, the collection of parts wheeled away to my van and I was now the proud owner of what would become the chassis for the V-Twin project.

As a point of somewhat academic interest, this chassis continued to be used after AJS had come into the ownership of the Collier Brothers. The picture below is a 33/7 Trophy Model (a 1933 350cc off-roader) that is awaiting its turn for restoration in my workshop and this has a nearly identical frame, forks etc.

The first question, of course, is: “would the engine and gearbox fit?”. At the time, the crankcases were set up on the milling machine and so the spare pair of crankcase castings and gearbox were positioned in the frame, very approximately, to see if they had any chance of squeezing in. Although the picture below is somewhat blurred it seemed that they would probably fit although space would be tight.

Although I didn’t know at this stage what gearbox I would eventually use, there was a heavyweight 4-stud Sturmey Archer gearbox included in the bits and pieces. The gearbox casting unfortunately had damaged threads for the main 3/8” studs but, internally, all the parts were in excellent condition. I have repaired quite a few of these early gearboxes in the past by filling the holes with Lumiweld and then re-drilling and tapping the threads. The process that I have evolved is:

1)    Having stripped and cleaned the gearbox, scribe lines on the top to indicate the centres of the holes. These can be picked up on the milling machine later to position accurately the hole centres. Incidentally, the distance between the holes are always “round numbers”, in this case 3” and 2 1/8” between centres

 2)    Drill out the holes to get a clean internal surface

 3)    Clamp a piece of stainless steel plate to the underside; this is to stop the Lumiweld pouring out the bottom.

 4)    Preheat in an oven to ~250 0C. This isn’t strictly necessary but it simply avoids heating up the whole structure with oxy-acetylene

 5)    Heat around each hole in turn with oxy-acetylene with a large nozzle (#7 is about right) and, when sufficiently hot, melt a small piece of Lumiweld into the bottom of the hole. Stir this around vigorously using a stainless steel rod until the Lumiweld starts to adhere to the aluminium surface. Melt in more Lumiweld and work upwards in the hole making sure it sticks to the surface. Extend the molten Lumiweld beyond the top of each hole as it will form a crater as it contracts upon cooling – like a casting.

 6)    Machine off excess material and drill and re-tap the holes.

 The pictures below illustrate the process

Later in the project, I was pleased that I had made this repair as it transpired that this was the only gearbox that would actually fit in the space available.

No comments:

Post a Comment