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Coil Upgrade

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  • Coil Upgrade

    Hi Folks,
    Thought I’d share what I’ve been doing in regards to a coil for my 31V Harley. The bike came with only one of those round Jap coils from the 70’s but since I’m restoring I want to go the OEM look.
    I was discussing my dilemma with fellow forum member ‘downunder vl’ and he put me onto a company here in Oz that makes 6v dual lead coils and kindly donated a dead OEM unit so I have an original housing to use.

    They also have a 12v version if anyone is interested.

    So, after gutting and cleaning the OEM housing I started pondering and here’s what I came up with.
    I got some 5/8” square steel bar, cut a piece at 4 ” long, and using the rivet holes in the housing as a guide marked out, and drilled to 3/16”, the 4 holes to allow the rivets to pass through when reassembling.
    Then using the new coil mounting holes as a guide drilled the other 2 holes (at right angles to the rivet holes) and ran a ” UNF tap through them.

    Square Bar.jpg

    Then some more measuring and I jumped on the lathe and made the 2 aluminium bushes shown in the pic below. I used aluminium because I had some. Steel, brass, plastic, anything should work just fine.


    The length I made the bushes keeps the coil from contacting the steel bar and puts the plug leads in the right spot to go straight up to the holes in the lid. But only in that plane. From the other they don’t quite line up but not to bad.

    Leads to Lid.jpg

    The front plug lead is too short for a VL but as they just screw in to the coil anyway I’ll change them out for OEM copies before final assembly.
    Once I mounted the coil to the square bar I sat it in the OEM housing for a trial fit. As you can see there is clearance all round.


    I used 3/16” bolts through the rivet holes to hold it all together but it occurred to me that once it was rivetted back together you’d have to unsolder the top to change plug leads and the bottom as well to change out the coil as you’d need to access the bottom ” bolt.
    So I made another length of 5/8” square bar but this time instead of drilling the 4 rivet holes to 3/16” I threaded them to 3/16”, got some 3/8” long button head cap screws and screwed 4 in from each side.

    Front View.jpg

    See next post for more.
    Last edited by Peter Cooke; 11-07-2020, 09:38 PM.

  • #2
    Rear View.jpg

    This would allow you to only need to remove the top in order to change leads or the whole unit.
    Not exactly OEM but once it’s all painted black the button heads will kind of look like rivet heads and they’re reasonably well hidden. Especially the rear ones.

    “Looks good from a galloping horse” as they say.

    Next up is the primary leads. Stephen from Tri Spark recommend 5 amp wire so I got some of that, some insulated crimp terminals and some plastic tube to go over the wire for extra protection.

    Primary Lead Bottom.jpg Primary Lead Top.jpg

    The other option is to solder the wire to the coils terminal and use some heat shrink to protect the joint but I was thinking along the lines of being able to slide the coil out and more easily replace it if it died. Either way should work just fine.

    That’s as far as I’ve got at the moment. I thought I’d put it up feedback as there’s always room for improvement. Thoughts please?
    Last edited by Peter Cooke; 11-07-2020, 09:41 PM.


    • #3
      What a fantastic upgrade, modern reliability and replaceable. I have gone the route of electronic regulators hidden in the original housing. This conversion looks like another big step forward for riding reliability. I see looking at their 12v coil it is 3.5 ohms, I wonder if that would work with points or only electronic ignition? Thanks for sharing and supplying the link to the coils.
      Bob Rice #6738
      End of the Line 1988 TW


      • #4
        Interesting!! Thanks for this.

        AMCA #6520


        • #5
          i did a similar conversion putting the 6V Tri-Spark in a '27-type coil housing. the HT wires leading from the Tri-Spark are not long enough to reach the front cylinder. Do you have that issue with the HT wires on the Tri-Spark you are using? i intend to use an NGK coil wire connector to get the length i need when the time comes to going back to an original appearing coil. i will put the connector under the tank where it is not readily seen.

          Steve Swan

          27JD 11090 Restored

          27JD 13514 aka "Frank"


          • #6
            Steve, the leads just screw in. Take them out and screw in the OEM type at the correct length.


            • #7
              Really. Wow. i guess i'll need to take a much closer look. The leads looked epoxied in on the Tri-Spark i received, but indeed i shall have a closer look. Thank you, Peter.
              Steve Swan

              27JD 11090 Restored

              27JD 13514 aka "Frank"



              • #8
                Steve, There is little epoxy or something there, I'm assuming just enough to stop the lead vibrating loose, but you can undo them.


                • #9
                  For anyone feeling so inclined...



                  • #10
                    Originally posted by Peter Cooke View Post
                    Steve, There is little epoxy or something there, I'm assuming just enough to stop the lead vibrating loose, but you can undo them.
                    Bravissimo, Peter! i turned the HT wires in the opposite they were turned in an wa-la! so i turned in HT cloth covered wires. Thanks for sharing your experience. i used a couple little pieces of aluminum "L" pieces and mounted my Tri-Spark coil inside the repro shell, Perfect! Without going into detail (read between the lines) had a "moment" where i attempted turning out HT wires on another coil that looked and felt by all intents and purposes the wires would come out... so i after that experience when i saw the wires in the Tri-Spark i decided to not press my luck x's 2... That's what i get for not putting on my 6x power magnifiers!
                    Steve Swan

                    27JD 11090 Restored

                    27JD 13514 aka "Frank"



                    • #11
                      No problem at all Steve. Everyday's a learning day!