Magneto air-gap

Cut a 5/8 inch wide strip of cardboard from a cornflakes packet or even a business card, as they seem to measure in at around 0.5mm.
Slot the cardboard in between the magnet and magneto arms, then turn the magnet (using the nut on the end of the crankshaft) so that the cardboard is trapped between the smallest space.
Once this has been done, simply tighten up the 4 magneto coil screws. The air gap has now been set.