Working on a Burman or AMC gearbox on the bench is, to be truthful, a bit of a pain. The lumpy and irregular shape means that it's unstable and rolls around. There isn't a flat face for it to sit on. If you've got a big vice that will open up to more than 3.1/2" then you could hold the bottom stud mounting in that, but it's not ideal and you risk damaging the cast aluminium case. Quite honestly, if you've got access to a bike lift, it's easier to work on the box if you leave it fitted in the bike, but if that isn't an option, then why not make a stand to hold it steady while you work on it? I decided to do just that. A root around in the garage and I'd come up with most of what I needed... a pair of old rear engine plates, some 2" x 1" rectangular steel box section tube and a couple of old gearbox mounting studs.
I bolted the gearbox to the engine plates as it would be when fitted on the bike. I could then mark where to cut the plates to remove the portion that I didn't need... the bits that bolted to the engine. The surplus parts were cut off using a 1.2mm thick cutting disc in my angle grinder. Having done that, I could measure up for cutting the rectangular tube. It's really simple, a vertical length, a horizontal length, and two shorter bits to act as feet. Actually, I decided that I needed another short length to act as a strengthening support for the vertical tube as that would be carrying the full weight of the gearbox and engine plates. The feet would be attached by screws as it would be easier to store on the shelf with the feet removed when I wasn't using it. The main parts, I brazed together using flux coated "SIF Redicote No. 1" silicon bronze rods. They flow easily at the temperature I can reach with my oxygen/MAPP gas torch and make a really strong joint. As the engine plates needed to be 3.1/2" apart to match the gearbox mounting bosses, I would need some spacers as the rectangular tube is only 2" wide. Some 20mm diameter bright mild steel bar in my scrap box would do nicely. Half an hour or so on the trusty Myford lathe and I had all six made. I'd decided to bolt the plates in place rather than fix them permanently. That way, it was adaptable should the need arise in the future. I'd got some M8 rivet nuts and an installation tool, so I set six into the sides of the vertical tube, three on each side. The spacers were recessed on one side to fit over the rivet nuts. The engine plates were drilled to suit. I set M6 rivet nuts into the tube I was using as feet. The cap head screws that hold it all together are stainless steel.
I thought that was it, all done, then I had another thought. The big nut that holds the final drive sprocket can sometimes be a real pain to undo. With the gearbox out of the bike, there is no easy way to hold the sprocket and stop it from turning. Using a chain wrench is really the only option and that makes it an awkward, two handed job. Could I come up with something better? Damn right, I could. Another shoogle in the box of worn and broken bits I've taken off various bikes and I came up with an old final drive sprocket. That would do nicely, thank you. If I could fix that to the back engine plate, then with a short length of chain, I could hold the gearbox sprocket and stop it turning. Back on the lathe, I turned up another spacer to fit the old sprocket and bring it into line with the one on the gearbox. The spacer needed to be welded to the old sprocket. Unfortunately, at this time I don't have a MIG welder... but I know a man who does. For the price of a pint, the two parts were securely stuck together. Now I just needed to drill and tap the spacer and drill the engine plate to suit. Three high tensile M6 cap head screws hold the sprocket in place on the rear engine plate. This time, it really was just about finished. A trip down to see Richard at Breckland Finishing to arrange for all the bits to be powder coated was the final task. When I got the bits back a week or so later, all I had left to do was knock in some plastic blanking plugs to finish it all off. That's it, the job's a good 'un and I'm happy with that!!
Last updated 06/02/2021