It is very important to maintain the contact breaker points gap at the size recommended by the manufacturer. The recommended size usually covers a small range. Most common sizes are in the range 0.012"-0.015" but ML used 0.010" on their Maglitas and Eisemann specified 0.020" for their AM magnetos. Selection of a particular figure would have been based on the physical construction of the magneto with particular regard to:
As the magneto rotates, there is one definite position where the best efficiency is obtained. This is just as the edges of the armature move away from the pole pieces causing the magnetic field to collapse. It is at this position that the fibre heel on the points needs to have reached a position on the upward slope of the cam to start to open the points. This is known as the internal timing of the magneto and most manufacturers provided a means of fine tuning this position. It would have been adjusted to the optimum position as part of the manufacturing process and would not normally need to be changed unless parts such as the cam ring were replaced. Adjusting the points gap by moving the fixed half of the contact breaker points will cause the points to open when the fibre heel is higher or lower on the rising edge of the cam - closing the gap retards the timing, opening the gap advances it. In short, adjusting the size of the points gap alters the internal timing of the magneto.