Safety Gaps

If the plug cap falls off the spark plug on a single cylinder engine, the engine will stop pretty quickly. Multi cylinder engines are a little different. If a plug cap falls off on these engines, the engine will continue to run on the remaining cylinder(s). It will obviously not run as well as it should but it will keep going. This means that the loose HT lead will continue to be supplied with a spark voltage but, with no spark plug connected, the spark will try to jump to earth by some other route. This may well be through the insulation somewhere and a continuous stream of these sparks is very likely to cause permanent damage. To prevent this, all multi cylinder magnetos incorporate a spark safety gap. The HT current will always follow the easiest path and, under normal operation, that will be via the spark plug. If that path is not available, the next easiest path is via the safety gap. This route will require a higher voltage than that needed by the spark plug but the size of the safety gap is selected so that the voltage needed to jump the gap is lower than that likely to cause damage to the insulation. Here, 'insulation' means the insulation used on any or all of the HT coil, pickups, slip ring, rotor arm, distributor, HT leads and caps.

Safety gaps come in different forms and are positioned on the magneto in a variety of different places. In all cases, one side of the gap will be attached to the HT circuit, the other will be connected to the magneto body which is earthed. Here are some examples of safety gaps:

This Bosch ZEV armature shows one way of including a safety gap. When the winding of the secondary coil is completed, two lengths of tinned copper wire are soldered to the end. One wire goes to the slip ring and eventually to the spark plug. The other wire, shown here, wraps around the coil under the insulation layers. It protrudes through the insulation and points towards a pip on the brass end of the armature. The armature body is of course earthed via the carbon earth brush.
This Lucas K2F magneto shows the view through one of the pick up holes. The safety gap is made using a pointed screw screwed into the body. The point on the screw is directed towards the brass segment on the slip ring. There will be a second pointed screw on the other side for use with the second cylinder's HT circuit. This is a very common method used on twin cylinder magnetos from many manufacturers. Washer(s) under the screw head will affect the size of the safety gap. Make sure they are not forgotten on reassembly or the spark may occur here rather than at the spark plug!

On the twin cylinder Bosch Type D2 magneto, the spark gap is built into the pickups. This picture shows an assembled pickup together with the component parts.

The safety gap used on multi cylinder magnetos which incorporate a distributor Is often located between the pickup and the body, underneath the arch of the horseshoe magnet.
Another location for the safety gap on multi cylinder magnetos is on the rotor arm underneath the distributor cap. This one is on a spark gap distributor fitted to a BTH GA4 magneto. The gap from the rotor HT terminal to the HT lead terminal in the cap is obviously considerably less than the size of the safety gap.

This picture shows the safety gap on a Scintilla GN6 magneto.

The way in which a spark is made to appear across a gap is described on the Voltage at the Spark Plug page. The ionised gas in a gap  acts as a conductor thereby allowing current to flow through the gap. For the spark plug gap, the ionised gas is immediately dispersed once the fuel/air mixture is ignited and the power stroke is under way. Under normal operation, no sparks occur at the safety gap but if they do, the ionized air can remain in the gap. This can mean that, when the next spark comes along, the path through the safety gap is an easier route to earth than that provided by the spark plug. This would result in a misfire.  Stationary safety gaps would be the most  susceptible to this problem. Safety gaps which are always moving such as those mounted on the armature or the rotor arm are better as the air in the gap is continually changing. A misfire caused by a safety gap sparking is not really a major problem in itself. If the safety gap is in use, it is an indication of a problem elsewhere which will need to be investigated and corrected anyway. 

The actual size of the safety gap is chosen very carefully. Too small and the spark may occur at the safety gap rather than at the spark plug. Too large and, if the route through the spark plug is not available, the insulation may be over stressed. Period literature for the various magnetos sometimes give a recommended safety gap size. Examples for two of the magnetos shown above are as follows:

The Lucas K1F/K2F/KVF literature  indicates a safety gap from the point of the safety screw to the brass segment on the slip ring as 6.5 to 7.5 mm.

The BTH AG4 literature indicates a safety gap of 8 to 8.5mm.