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John Deere Tractors Discussion Forum
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Duane Larson /? on coil air gap

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34gpa

12-07-2007 06:58:39




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I was reading the thread from earlier this week and saw your post, and was wondering if you could expand a little on setting the proper air gap on a magneto. Thanks for your expertize




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Duane Larson

12-07-2007 18:44:13




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 Re: Duane Larson /? on coil air gap in reply to 34gpa, 12-07-2007 06:58:39  
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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John T

12-07-2007 10:01:55




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 Re: Duane Larson /? on coil air gap in reply to 34gpa, 12-07-2007 06:58:39  
34, Until Duane returns to explain the method he uses to set that gap (Hes a ton more precise n detailed then myslef) I will lay a little theory on ya of the what n why. As I recall he and I had this discussion at the last GOG

In the Wico Mags (others for that matter) the maximum coil primary current occurs just as the rotors magnet is exiting the iron field laminations (0.090 just passed/outside) and thats when you want the points to break open in order to maximize the energy storage/transfer which eventually gets discharged across the spark plug gap. Now when alllll was new and in spec, especially the rotor bushing/bearings and more especially the points cam follower n cam lobes, that was achieved when the points were gapped on high cam to that 0.015 setting you hear about on Wico C and X mags..... ...

Buttttt tt as the points cam follower wears down and/or the cam or bushings wear, when you adjust the points at that 0.015 on high cam they may or may NOT still be breaking open just as the magnet exits 0.090 past the iron field laminations (i.e. at max coil primary current) sooooo ooo the edge gap setting (he can explain adjusting) is really the preferred method of points adjustment over the high cam 0.015 UP UNTIL THE POINT the cam follower or other wear is so extreme the points gap (say around 0.012 or so) or dwell is no longer within tolerances i.e. you need a new set of points (most typical) or theres a cam or bushing problem.

He as a professional rebuilder (and perfectionist in my opinion) likely sets the points using that method (or checks that against the 0.015 at least) versus the casual rebuilder (like this dude I know lol) who uses the 0.015 alone.

I also am very interested in his method as Ive tried using an ohmmeter or test continuity light to set the points to just break open at precisely that edge gap time, but it dont seem all that accurate n perfect to me as loosening n tightening the points down in place can make things move ever so slightly.

Of course, mags other then the Wico C n X can have different specs n settings but the theory is the same so I look forward to his answer with you

John T (Only a Nerd Wannabee, so will shamelessly stand corrected if the above is incorrect)

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Duane Larson

12-07-2007 08:03:24




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 Re: Duane Larson /? on coil air gap in reply to 34gpa, 12-07-2007 06:58:39  
34gpa,
I'm headed out for awhile, I'll try to reply this evening about the "edge gap".



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