I had a stimulating phone conversation with a well-known Harley historian tonight. We covered the bases, so to speak, in an hour's conversation, but what I want to report is what came up in one part of that conversation. He axed me what was up with repro coils. He told me that one theory someone proposed to him related to engine heat, and that Flatheads, with more top end heat, seemed to fry repro coils before OHVs, based on anecdotal evidence.
It turned out that it has something to do with heat, but it turns out to be heat at the sparkplugs' tips, between the center electrode and the ground electrode, and two electrical principles and coil polarity, as it relates to a pair of electrical theories: the electron theory and the theory of thermionic emission, which are factors in automotive wasted spark ignition. (AKA: "Why do Tedd's V-Twin repro coils so often fail prematurely?") All this discussion of direct current, alternating current, electrons, heat and current flow stimulated my wetware, and between what he said and what I said, after we hung up I came to more completely understand what I thought I knew "for years."
The Answer: Basically, the coils fail because their primary-coil is wired backwards from the direction intended in the designs of OEM Harley 6v coils (1930-47, and the next, 1948-64), and yes, the locations of the "+" and "-" terminals do matter.) If you buy a dash-30 (uses two u-bolts) or dash-48 (bolts top and bottom to blocks welded on the frame seatpost) repro ignition coils, DO NOT attach your power wire from the main switch to the "front" terminal (as the coil is mounted on a bike) on the coil, and your points wire to the back terminal. This is the recipe for disaster (early failure).
The primary winding within the repro coil (about 220 turns of wire) is "thrown" in the opposite direction/rotation within the box from the OEM coils that used to be supplied by the MoCo. The secondary, "hi-tension" coil (about 12,800 turns of extremely fine wire that connect to your sparkplug wires) are excited in the opposite direction ("+" and "-") from the way they were designed, if you follow the old manuals and wiring diagrams when installing your repop coil. This creates an opposite polarity at the sparkplugs from the way sparkplugs normally like to be treated, and leads to their early demise, if not that of the coil, itself.
And after we got off the phone, I did a little internet research and got the details and kerrect terminology:
This is the basic diagram of a Harley "wasted spark" ignition system
What you have to do is connect the switch 'ign hot' wire to the back low-tension terminal on the repro coil, opposite of the diagrams in Harley Service manuals, and opposite of the way it was done on the bikes originally, and attach the wire that connects to the timer/the points, over on the opposite side of the bike, to the front terminal of the coil, on the side facing the fork and front wheel.
If you examine a recent Tedd's repro coil, you may see a tiny "+" embossed in the tin case on the back, near the terminal, but it is a fairly recent addition, and is not publicized anywhere. It's only essential to the life and reliability of the coil, is all.
All seem to agree: Tedds' coils fail because polarity does matter, and backwiring leads to eventual sparkplug failure. Yup.
Perform the "lead pencil test" described above, in the third reference, the "International Harvester Manual," & shown on "Next Page: Condensers" at its top right. Do it under low light, and check your current polarity at the plugs. On a UL, you can use a tiny c-clamp to hold a wooden pencil alongside the adjacent cooling fin and the lead point will be in the perfect position for the test, right next to the top of the rear plug, with the rear plug wire removed and placed right on the opposite side of the pencil. The spark should appear between the plug wire and the pencil, and the 'flash' of an orangish color, between the pencil and the terminal on top of the plug. Look quick, for the spark on one side and the "flash" on the other. It's quick. You can snap "closed" points if you're using them. I had to kick over my electronic ignition and make it turn to fire off the coil.