Rewinding A Magneto Ignition Coil

[Vintage Engine Repairs]

Ever wondered which end of the primary winding the secondary winding connects to on a vintage engine magneto coil? The ground side, or the points side? I doubt it, but I did…

I searched old manuals, ignition books, rewinding guides and magneto resources. Most information refers to battery ignition systems, not magnetos. The people I emailed or called either didn't know, weren't willing to say or didn’t reply. Nobody seemed to provide a clear answer.

Over several days, I carefully unwound multiple original coils, snapping wires each time. Finally I managed to preserve the incredibly fine secondary wire where it joins the primary on one!. Anyone who's worked on these knows that wire is finer than a hair and breaks if you so much as look at it the wrong way!

I finally found the answer: the secondary winding connects to the points (hot) side of the primary winding.

One thing this journey reinforced for me is how important it is to share knowledge. Too many old skills disappear because information gets guarded, lost, or buried away. These old saws, engines and magnetos won't keep themselves alive.

To help preserve the knowledge, I'm going to film a detailed step-by-step video showing the complete process of rewinding a vintage engine ignition coil from start to finish, including winding direction, insulation, connections, testing and final assembly. Maybe one day you’ll need to rewind one if your coil is NLA - I hope it simplifies the process for you..

If we don't pass these skills on, they'll disappear with the people who know them. Picture at 1am after hours de- potting a second coil and using a hand lens to trace the wire. I’m absolutely exhausted but I bloody found it - I snapped it, like the others, but enough remained that I could see it - I win…

 

[el33t]

EDIT:
Unfortunately, in the version of the coil that Tom disassembled, the high-voltage output is permanently connected to ground via the primary winding, which has a negligible resistance compared to that of the secondary winding, so without prior knowledge of the exact resistance of the secondary winding, it is indeed impossible to determine which version we are dealing with using only a simple ohmmeter without disassembling the coil.

 

[Andrewengineer]

I last did all that a few times about 25 years ago. I don't think it really matters to which end of the primary the secondary is connected. A few volts added to or subtracted from the thousands created by the secondary are neither here nor there. I sourced my wire from scrap HT coils, unwinding it carefully on to a spool (using a lathe). The main lesson I learned was to begin by making a torque limiter (a simple friction device driving the coil) on the winding (or unwinding) machine (lathe). Second was how to join breaks in the the hair-like wire during winding. Hold the broken ends between finger and thumb, half an inch or so projecting. Twist the ends together tightly, cut off some of the twisted pair leaving perhaps 3mm holding the wires together, move the end slowly into the side of a fine propane flame. The insulation burns off and the ends melt together forming a little ball. Carry on until you are sure the ball contains both wires. Allow to cool (about a second will do), and gently pull the joined wire stretching out the joint. There will now be an uninsulated ball. Carry on winding the coil, but put a scrap of thin paper under the joint as it is about to reach the coil, and another over it before the next turn comes alongside it. (Or fold a scrap of paper over the join just before it joins the coil.) Third: don't try to achieve a neat layer by hand-guiding the wire on to the coil; set the lathe feed to just over the wire thickness. Fourth: brush varnish over each layer and then wrap in a layer of thin paper. Fifth: You will not be doing this to earn a living; it will be for interest, satisfaction or learning, time spent not being a consideration. Good luck with your restorations.

 

[el33t]

I last did all that a few times about 25 years ago. I don't think it really matters to which end of the primary the secondary is connected. A few volts added to or subtracted from the thousands created by the secondary are neither here nor there.

I know you probably used "a few volts" as a figure of speech, but for the record it’s more likely to be a few hundred volts from the voltage induced in the primary winding. But really, it doesn’t matter much.

It seems to me that the way the connections are made in the coil that Tom found simplifies the design - at least one connection/terminal fewer.
The downside is increased insulation requirements.

I don't know what materials Tom looked at, but I quickly found in the Homelite and Stihl manuals that they only describe one connection method: the one where both windings are connected to ground. Here's an example from the Homelite manual: