In the majority of applications, two magneto bearings are used as a pair. The outer races are pressed into the magneto housings so that the open sides of the races face each other and the inner races are pressed onto the armature shaft. The cage with the balls can then be replaced onto the inner races. The armature is placed into the magneto body, thereby assembling one bearing. The end housing with the other outer race is then placed over the armature shaft and into the magneto body thereby assembling the second bearing. Keeping a constant check on the armature to ensure that the bearings are not over tightened, the end housing screws/nuts are put in place. If the bearings do start to go tight because there is not enough end float, a large shim is placed between the magneto body and the end housing to move the outer races further apart. If, once the screws/nuts are done up tight, there is too much end float, the armature is removed and shim(s) placed under one of the inner races to move the inner races further apart. Once the correct end float is achieved, the armature is removed again so that the bearings can be greased before everything is assembled for the last time.
Correct end float in a magneto is an important consideration. Tight bearings would obviously be a problem but too much end float can also create problems. Loose bearings will allow the armature to ‘float’. If it is excessive, this will allow the armature to rub on the pole pieces as it revolves. It will also affect the timing as the CB points (rotating coil magnetos) or the cam (rotating magnet magnetos) will be bouncing about resulting in variable ignition timing and erratic running. Too much end float is especially a problem with magnetos using a face cam arrangement to open the points as axial movement of the armature will change the points gap setting and therefore the timing.
We always assemble magnetos with ‘next to nothing’ in the way of end float - just perceptible play of one or two thou. When they are working, magnetos will inevitably get warm and as a result, they will expand a little. The aluminium body will expand more that the steel/brass armature so, as it warms up, end float will increase slightly.
Magneto bearings are usually lightly loaded and so will last a long time if kept adequately lubricated. However, they do wear and this will show up as increased end float. The end float could be re-adjusted as outlined above but this would be false economy. Adding a shim to the armature would appear to take up the end float but the balls will not be seated properly in the grooves – the end float will soon re-appear.
Magneto Bearing Dimensions
The descriptive names listed below are the most common and are used by several different manufacturers. However, some manufacturers use/used different markings. For instance the E15 bearing may be found with FB-15, E-15B, ND-15, N 15 and, no doubt, other variations too.