CLASSIC TECHNIQUES

Finding a specific sprocket for an unusual old bike can be a tricky business. If you have an old one to use as a template then it’s possible to make your own, as John Lay explains…

Icouldn’t find a front sprocket for my 1954 Moto Guzzi Cardellino, a 75cc two-stroke single. So I set about making a replacement sprocket. Firstly you need to determine the size and type of chain, type 415 in this case (or ½” x 3/16”). Next, track down a suitable blank. These are also known as BS pilot bore platewheels. You need to order one with the correct number of teeth and plate width. Finding one the right width proved to be difficult and in the end I used one that was too wide, although it had the correct pitch and number of teeth. Fortunately platewheels tend to be ‘tough’ rather than ‘hard’ and are therefore machinable. As the sprocket needs to have its centre bored out to suit, it was no great effort to turn it down to the required width at the same time.

Before this can be done, some means of holding the platewheel needs to be addressed – trying to hold a toothed sprocket in a three-jaw chuck is never likely to be a success. To this end a split sleeve or carrier was made. This was turned up from a piece of aluminium bar, with a ‘pocket’ turned so the sprocket would just fit inside. It was parted off and a saw cut made on one side.

The sprocket is placed in the sleeve and then into the chuck. When the chuck jaws are tightened, so the sleeve contracts, holding the sprocket firmly and square. Five holes were then drilled. The old and new sprockets were mounted on a stub mandrel, so the old

sprocket could act as a guide. With the holes drilled and tapped (5mm x 0.8mm), the two were bolted together to keep them in line with each other. The old one acted as a guide as before, but this time to form the cut-outs of the spline. The cut-outs were made using small square and round files.

Once done, the new sprocket could be fitted. Note the purpose of the five tapped holes. These are to allow the fitment of a ‘lock’ screw to engage with one of the notches in the securing ring to prevent it coming undone. Finally, before fitting the sprocket it was de-burred and the tips of the teeth given a slight chamfer to match the original – these were lost when reducing the width. And now the Guzzi can go back on the road!

The split sleeve ‘carrier’ and the old sprocket

The new sprocket in place, with its locking that engages with the securing ring

Left: Using the old sprocket as a guide when forming the spline cut-outs

Right: New sprocket being drilled using the old one as a guide