Points Gap

Points Gap

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:
  • The position of the edges of the armature core relative to the pole pieces
  • The position of the fibre heel of the contact points relative to the rising edge of the cam
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.
If the internal timing is advanced too much (by having a large gap) the points may open before the field has collapsed – resulting in no spark at all. The physics of a magneto's method of operation means that there is some automatic advance as the revs go up. It may be that a magneto with too much advance will spark at cranking/kickover speeds and low revs but this additional automatic advance as the revs increase may be enough to lose the spark altogether at higher revs.
If the internal timing is retarded too much (either by having a reduced gap and/or by operating the advance/retard mechanism) the rate of magnetic field collapse will have slowed down by the time the points start to open so the spark will be less intense. Magneto efficiency increases with speed of rotation so this will be less apparent at higher revs but on a 'tired' magneto starting may be difficult and it is not unusual to have no spark at all at full retard at cranking/kickover speeds. (Watford, Splitdorf and Scintilla, amongst others, found a way to get round this but that's another story).
So – to get the best performance out of your magneto, stick to the
manufacturer’s recommended points gap! 

NOTES
1: When setting the points gap on one side of a twin cylinder magneto, it is not uncommon to find the size of the points gap on the other side is different. This is usually put down to wide manufacturing tolerances. Although not ideal, this in itself is not a major problem - see our Points Bounce page for the reasons. What is important is that the positions in which the points start to open on the two sides are the correct angular distance apart. How to check this is shown on the V twins page.

2: All of the above references to timing are concerned with the internal timing of the magnetoThis is totally different to the ignition timing of the engine which is all about making sure the points open at the correct position of the piston as it approaches the top of the cylinder bore. Yes, the ignition timing could be fine tuned by adjusting the size of the points gap but doing so would sacrifice magneto efficiency and is not recommended.  
Share by: