I have a 1937 16/70 which has a speedo that doesn't work. The speedo drive is an annular worm gear that is on the splined output shaft. It engages with a pinion on the end of the speedo drive cable. On my car, the annular gear is missing from the gearbox. I have stripped it down to fit a gear I have from another gearbox, only to find that the shaft on mine is a couple of mm too thick, though it has the same number of splines.
A few questions:
> does anyone know where I can get the correct part from?
> If not, does anyone know an engineering shop that could make the internal diameter of the gear larger so it fits the shaft of my gearbox?
> If I do that, am I right in thinking that the external diameter of the part I have would be the same as the original fitting?