Magneto Drive Gearbox

Magneto Drive Gearbox
Magneto Drive Gearbox
Magneto Drive Gearbox
Magneto Drive Gearbox

I was asked to machine a set of special gearboxes for MG vintage cars. These cars, originally made in the mid 1930's, are being restored in 'racing' form.

Why a Magneto?

The racing version of these MGs needs another type of ignition system to deliver the spark to the spark plugs that keep the engine firing. Most spark ignition cars (even modern cars) rely on the battery charged by a generator, running off the engine to deliver the power for the spark. On a racing car, most of that system is not needed so why not generate the power for the spark directly from the engine and feed it straight to the spark plugs? The type of generator that can do this is known as a magneto. The engine will be started manually, with a crank handle, and only stops when the racing ends. The magneto generator fits in place of the distributor (the device that distributes the spark to the right cylinder to fire the engine in the right order). Unfortunately, the magneto is about three times the size of the distributor and so has to be mounted off a special gearbox, driven by the original distributor gears at a right angle on the side of the engine. These gearboxes are very rare and it had been decided to cast new gearbox casings and collect the internal parts together to make them. 

No drawings

So I was presented with all the internal gearbox components and an unmachined gearbox casings! Drawings are a luxury for me, so nothing new here. However, the positioning of the shafts in the gearbox duriing the machining required me to make up a dummy bearing. This enabled me to set the main shaft in the gearbox and set the gears at the correct spacing during the machining.

Oil Drilling

Just to make life a bit more interesting, the gearbox casings needed a drilling that would guarantee oil would feed to the plain bearing on the input shaft. This bearing is just visible  as the 'coppery' (it's actually a bronze bush, one inch deep) ring at the end of the final machined casting. This required a drilling at the side of the upper roller bearing. Then I was able to 'break into' this hole using an extended small diameter milling cutter.

This gearbox delivers a racing ignition system for a heritage racing car.