34gpa, John T has pretty well covered the issue of "edge gap". To summarize the salient points - it is clear that having the points open when the trailing edge of the magnetic rotor has left the laminated pole faces by the distance called the "edge gap" provides the largest primary current and the "best" voltage output. However, setting the edge gap is not something most folks could (or would) do reliably, and the magneto design folks realized the proper edge gap distance could be realized by a secondary method, and that is setting the points to have a specified opening - and often in the early days wiring a point-setting piece of metal to the magneto helped ensure the proper point gap and hence edge gap.However, as things wear the point setting often no longer provides a good approximation to the edge gap. Magneto repair (to me at least) includes an analysis of the point setting vs edge gap issue, and often replacing worn bushings and/or points will bring them back into correspondence. However, sometimes it will not, particularly if the cam or point pivot pin is worn. I always set the edge gap, using a small drill bit of the proper diameter. An ohmmeter connected between the movable point and ground (before the coil primary or condenser is connected to the movable point) will accurately tell when the points are just touching at the proper edge gap. And yes, John, it can be a bit touchy getting the stationary point fixed in place without it moving, but a small flat washer between it and the screw head helps, plus holding your tongue in the proper position, and a good screwdriver. I then check the point gap and sometimes it is not close to the specified value. Since Deere's run relatively slowly (compared to an automobile), and the cam allows the points to remain closed for a relatively long time, the "dwell" (time the points are closed, allowing primary current to flow and a magnetic field to build) is not a particular concern. Sometimes it is possible to achieve a proper point gap and edge gap, but setting the edge gap and allowing the point gap to fall where it may does ensure you are getting the best spark available from the magneto. Some magnetos have separate adjustments for the points and edge gap, thus providing for the best of all worlds. The Edison Splitdorf CD comes to mind, and some early Wico magnetos.
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