Magneto Internal Timing

Magneto Internal Timing

In an efficient magneto, the points need to open at a very specific position of the armature - just after the armature leaves the pole piece. Rotating the armature slowly by hand, it will be noted that there are positions where the magnetic field is offering resistance to the turning motion. Turn a little further and the armature suddenly jerks forward as the magnetic pull gives up and lets the armature go. It is just after this position of the armature that the points need to open. This is known as the 'internal timing' of the magneto. It is important to understand this if parts are to be interchanged between magnetos. This page shows some of the pitfalls which can catch out the unwary!

This picture shows three Bosch points plates. Although they all look identical from the other side, they cannot be interchanged. The cutout is where the fibre heel is located but look at the position of the locating key on the taper. These will position the fibre heel at a different position relative to the armature so the internal timing of the magneto would be widely different for the three points plates.

These two BTH armatures are identical in every way except for the position of the locating slot for the points - shown in the 6 o'clock position. In this case, the two armatures use the same points and cam ring but a different cam ring housing which provides the required 90 degree difference in the points opening position.

In this picture of Lucas face cam plates, all have been placed so that the slot used to locate the plate in the magneto is shown in the 12 o'clock position. The red lines show the rising edge of the cam where the points start to open - all different! 
The top row shows plates with a narrow locating slot showing that they are meant for a fixed timing magneto. The others have a wider slot allowing the cam to be rotated a little to provide a manually operated advance and retard. A second slot on the cam plate is used to hold the manual advance and retard mechanism. Only one slot is required for this but there are usually two slots to cater for the advance/retard control to be fitted on either side. Even so, some of the plates have had extra slots ground in to allow fitting to other magnetos. In addition, the cam plate on the left has had it's timing slot widened at some point - probably in an attempt to adjust the internal timing by allowing the cam plate to rotate a little further than originally intended. Unfortunately, this type of modification usually results in the advance/retard control pulling out of it's cam plate slot. 
Just to compound the problem - it's not clear from the picture but in this selection of cam plates there are three different thicknesses...... 

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