Modeler's Corner

Coils and Carts

| September 2005


Hello again, everyone. I want to start by reminding everyone that the annual gas engine show in Portland, Ind., is right around the corner. If you think you can, you should drop everything you are doing and head to Portland: It is the biggest gas engine show in the country! You can get full details at

With temperatures running 90-plus degrees Fahrenheit and no rain for the past 45 days - but with typically high humidity - I have done very little in the shop this past month. I have, however, been working on a couple of items that I'll pass on, as they do have some relation to the hobby.

First, I now have over 2,000 feet of Essex Magnet Wire. This is the same stuff used in the automotive industry for winding coils. I have 18-1/2-gauge and smaller, and have thought about building a coil on my own - an easy task, right? Well, after a couple of phone calls and talking with actual coil experts, along with proofing Ignition Coils and Magnetos in Miniature by the late Bob Shores. (Bob's wife, Margaret, still sells these fine books, call her at (813) 645-8322.) I have found that maybe this is too much of a challenge and should be left to the pros.

I did find that a 3/4-inch center is recommended and with 18-gauge wire a total of 750 turns are needed to give a meter reading of 4-5 ohms at the 200 ohm setting. But, I have 18-1/2-gauge wire and want to reach a 6-ohm reading. My math works like this: Take battery voltage, usually 12-volt, and divide it by the ohms your coil allows; this is how much umph you get to your igniter. I want to get 2 volts to the contacts, and I found that even this little change in wire size has made a big difference in the turns the coil will need. I have over 1,200 turns on a 7/8-inch arbor filled with strands of soft wire and the coil still only reads 5.6 ohms at best. I used layer paper such as you might find in an old coil, separating every layer. This coil weighs in at 3 pounds, and still more wire is needed if I want to hit the magic 6 ohms.

Another thought I had was to use smaller coils and run them in parallel. I found that a coil with a 1-inch arbor with 235 feet of 18-1/2-gauge wire on it will give a reading of 1.5 ohms at a 200 ohm setting, so using four or five of these wired together would give you a 6-ohm reading. This did not work either, as I put five of the coils wired together and still only got 3.4 ohms. But why?

These coils are each on different strands of soft wire and maybe this is the reason, but still I have not finished making my coil. I want to fill the coil box with tar, plastic or oil when finished, and may even laminate a fine finish onto the plywood box. One concern here is that maybe I am using a 12-volt battery too big for my models, leading to premature burning of my points and igniter contacts. So, I have learned something from the adventure to help me in my model making, but now I wonder what the smaller gauge wire for a coil would look like, and will it work with the heat generated? I am sure I'll know more after I finish reading Bob's book, and get this coil done and put to use.