BSA Motorcycle Crankshaft Nut

BSA Motorcycle Crankshaft Nut

BSA motorcycle crankshaft nut machined from some scrap stainless steel bar for a local motorcycle enthusiast.  I have been pleased over the years to help people with particular tasks. A while ago a knock on my door revealed a gentleman with an oily rag in his hand. He had seen my workshop when he came to replace our large refuse bin (trash for our American cousins). “I don’t know if you can help………but I have a crankshaft nut for a classic BSA motorbike. Can you make one for me?”

The front and back views of the offending item.

The first part of my decision as to whether I could help or no depended on what materials I had in stock.  I have made good use of my local scrap/breakers yard and bought offcuts and other items in Aluminium, Brass and various steels.  As luck would have it I had a piece of stainless steel bar that cuts beautifully. It also was about 0.25″ over the size needed.  So I made him no promises, other than I would give it a try and see what I could do.

Machining the nut itself

After gathering all the dimensions I decided that a piece of the scrap stainless steel bar be turned.  It was critical that the internal threads match those of the existing nut. So the first job was to make a stub mandrel with the correct male threads. This would help when cutting the internal threads to make sure the nut was a good fit. Not to tight, nor too loose. With the threaded bar turned I made a start on the nut itself. It seemed to me that the most ‘interesting’ part of the project would be the radius inside the front of the nut.

My approach was to first drill then screw cut the internal threads. Once I had a nice fit on the temporary stub bar it was the outside diameter that was machined. I decided I would leave the notches and other indentations for the milling machine and get the shape sorted first.  The front curved inset was achieved by removing as much material as possible. Then a piece of tool steel  was ground to the required radius. The curved part was turned using the profiled cutting tool.

The slots were simply at 90 degrees and the nut held in my dividing head on the milling machine. It was then reversed and the other two slots machined.

Thoughts on the process

At the time when I made this nut it was perhaps the most interesting item I had made to date. Not only because of the threads being critical, but also this nut was going into some ones pride and joy (and also a very expensive item)bike  and the thought of it spinning around at high speed, made sure I did my very best work.  I also worried that when the nut was fitted to the crank that the crank itself wasn’t damaged.  I would have loved to have had the crankshaft itself so I would know the threads were good enough, but that wasn’t possible.

Finished the Crankshaft Nut – did it work?

I handed over the nut to the BSA ‘Nut” (sorry) and I nervously waited for news of whether it was good enough to fit!  It was several weeks before I heard that the nut had fitted and the bike was back on the road! It was also pointed out that the work had been a ‘lifesaver’ since the nuts themselves were as rare as rocking horse pooh and it was most important to get the bike finished.

Some months after making the nut I was searching eBay for BSA classic bikes when I thought it would be interesting to see if you could indeed buy a new one?

So you can imagine my disappointment (that will teach me to be more modest!) when I found several available, and one for only £5! I rechecked just now and there are not so many available, and the price has risen to £15, but they are still there.  Still, I have yet to find a ‘Stainless Steel’ hand maid one….so there 🙂

For some lovely pictures of BSA (and other) classic bikes please check out this site.