How To: Put New Teeth on a Sprocket
I’ve been remiss in putting a photo of the Excelsior on this site, after mentioning the various (worn out or rebuilt) parts of it several times. Here it is as of last summer. The mechanical restoration is nearing completion, then painting and plating will occur later this year.
The project for this weekend was to put new teeth on the gearbox output sprocket. The Excelsior has a cross over gearbox, with the input on the left, and output on the right side of the bike. The sprocket had some very worn teeth.
The sprocket has 4 internal splines which would be difficult to reproduce in a home workshop (without an EDM or custom broaches). So I decided to use the inner part of the sprocket and put on some new teeth.
When viewing the sprockets side by side, the wear on the teeth can be seen. To test your sprockets, you can look for a pronounced hook shape to the teeth, or wear on one side or the other of the sprocket (check the axial alignment of your chain run with a straightedge).
Don’t bother fitting a nice new chain onto a worn sprocket. The sprocket will destroy the chain fairly quickly, and then you’ll need to fix both the sprocket and the chain.
Now, how do we put new teeth onto an old sprocket? In his excellent book “The Vintage Motorcyclists’ Workshop”, Radco describes two methods shown below: welding and riveting. I decided to go a slightly different route. I used the stepped layout of the rivet style attachment, but brazed the sprockets together, and used pins instead of headed rivets. Flanges locate the two parts to be concentric and axially flush. The pins engage through flanges on both parts, which locks them together to transmit the torque. It is overkill for the amount of torque that this sprocket will ever see, but it wasn’t too hard to do.
I made a mandrel to mount the old sprocket in the lathe then chucked it up and cut off the old teeth. I also cut a step into the remaining hub.
I did the opposite cuts to the new sprocket to make them fit together.
After drilling some 1/4” holes and then quickly making some pins, I brazed them all together. A quick session with a Norton flap wheel and the part looked ok. You should try a flap wheel. It is essentially some small pieces of sand paper mounted to a grinding disc hub. They come in many grits and can be used to quickly remove paint, rust etc. Or they will grind down metal, if you choose the rougher grits.
Now the sprocket is ready for use. But the nut that locks it onto the gearbox was looking pretty scruffy. Previous owners had used a hammer and chisel (or worse, a hammer and screwdriver) to tighten and loosen the nut. Making this new part was pretty straightforward, much like the clutch nut on the other side of the bike.
Typical lathe operations illustrated below… The fun part was that this is an internal thread, and left handed too.
Old parts and new parts:
Installation with the proper tool. this one is somewhat universal over a small range, and fits several parts on the Excelsior, as well as some bits on the Velocettes in the shop.
The next step will be to make new teeth on the clutch wheel. Here is a photo and a short video of the first step. More info is in a later article here.