Fitting tight shoes
Is a special tool needed to fit a dynamo field coil?
Andy Reynolds asks if he needs the expanding jack tool shown in old books to fit a Lucas dynamo field coil or if simply tightening the retaining screw with an impact driver will do the job.
Rotating a coil of wire within a magnetic field creates a current in the coil, a stronger magnet giving more current. Similarly, current put through a coil of wire wrapped around a block of iron will turn it into a magnet, and the greater the current the stronger the magnetism up to the material’s limit or ‘saturation’ point.
Motorcycle dynamos employ both of these features. A metal shoe inside the body is energised by the field coil to make an electro magnet that provides a magnetic field that produces charging current in the rotating coil (armature). Varying the field coil current alters the magnetism and thus the dynamo output. The shoe squeezes the coil into place and runs very close to the rotating armature so when renewing the coil it is vital to make sure the shoe settles fully home so it doesn’t foul the armature.
The Lucas tool was rather like a small car scissor jack that expands to press the shoe home. I made my own version (above) but in practice you just need to make sure you wrangle the shoe into the coil so that it will not get pinched anywhere before fitting and that the shoe is settled comfortably with its curve seated to match the bore of the dynamo body. That achieved, you only really need to tighten the screw normally, although I would recommend a dab of Loctite. You don’t want this screw coming undone…