Points Bounce

Points Bounce

The contact breaker points on our old magnetos are mechanical switches, usually opened by a fibre heel running on a cam and closed by a return spring. That’s the simple explanation but the design of the system is actually a compromise.

From a mechanical wear point of view, the points would ideally open and close gradually and could be achieved by making the opening and closing ramps of the cam a gentle smooth transition. However, from an electrical point of view, the points need to open very rapidly in order to ensure that, by the time the condenser has done it's job, the points would be open enough to ensure that the low tension current could not jump the points gap.

When the points are closed, the fibre heel is clear of the cam so when it hits the rising edge of the cam, it is not unusual to find that the moving point bounces, the fibre heel jumps clear of the cam and the points open to considerably more than the set points gap. 
This picture shows an EIC magneto running at speed but the points have been made to 'stand still' by use of a stroboscope. The gap between the fibre heel and the cam as well as the resulting large gap between the contacts can be clearly seen.

Points bounce in itself is not a problem as the spark at the plug is initiated when the points first open. However, misfiring at high revs can often be attributed to points bounce. This is because the bouncing points are still open when the armature has revolved to such a position that the points need to be closed in order to build up current in the primary winding ready for the next spark. The two main factors affecting the amount of points bounce are the weight of the moving point and the strength of the return spring.

Early magnetos running at low speeds used relatively heavy brass points. As required speeds increased, most manufacturers used moving points made of aluminium to keep their weight as low as possible. In 1959, Lucas went a step further and redesigned the points used on their motorcycle rotating armature magnetos. Their claim was that this was done to reduce inertia in order to allow higher operating speeds. This picture shows early Bosch (brass), BTH (aluminium) and the later Lucas ‘low inertia’ types.
The amount of points bounce can be reduced by fitting a stronger return spring but this can create other problems. If the spring is too weak, points bounce will occur at low revs resulting in unacceptable misfiring. Too strong a spring will put excessive pressure on the points pivot post every time the points open. Luckily, period Lucas literature provided spring pressure data for their magnetos. The weakest at 18-24ozs included RF4, SR1 and 4VR magnetos. The strongest at 28-36 ozs included KN1, N1 and NC1 magnetos. Interestingly, the 18-28ozs pressure required by the common K2F magneto was achieved by using a pair of springs. When we need to measure spring pressure, we use the test rig shown in the following picture. The brass wingnut on the right is tightened up, pulling on the moving points arm until the Magneto Static Timing Light indicates that the points are just opening.  The spring pressure is then read off the digital display.
Often only riveted to the points back plate, it is not unusual to find that the points pivot post is worn to such an extent that the moving point is a sloppy fit, resulting in unpredictable timing. In a really bad case, the pivot can come loose in the back plate. Fitting a new pivot pin is a fairly common requirement when overhauling a set of points. On the left of this picture, a Lucas back plate is shown with the pivot pin riveted into place.On the right is shown a set of BTH points used on their aircraft magnetos. These incorporated  a much more substantial fixing using a larger bearing surface and three small steel rivets.

More information on the size of the points gap can be found here.
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