Roll Your Own Spark Coil


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The following antique gas engine repair tip will help you roll your own spark coil in one hour or less. Please don't waste your time and question my values or measurements, just rely on my experience - amateur radio license call sign WA4DVV - and get right to work.

Buy or use your own methods to secure a common carriage bolt that is 7/8 or 1 inch in diameter and 4-3/4 inches in length. Cut off the head of the bolt and leave on the squared part of the bolt so that you have the full 4-3/4 inches in length. Threads on other end of bolt do not harm so forget them and continue to work.

Purchase one-half pound roll of 18 AWG Single Beldsol Magnet Wire (number 8049). This should be your total cost to make the coil. Wrap the bolt with two layers of plastic electrical tape from end to end. Measure 3/4 inch in from each end and on top of the electrical tape make a mark at the measured points with Band Aid tape. Put one end of bolt in a vise with the mark even with the vise-jaws. Now take the coil of Beldsol wire and roll some off. Measure six inches from the end of the wire and put this measured point against the bolt tight against the vise-jaws. Proceed to roll the rest of the wire by hand tightly around the boll working toward the mark at the other end of the bolt. Now start winding a second layer on lop of the first layer and work your windings back to the vise-jaws. Keep your winding tightly spaced, but if some of the windings keep slipping down into the layer below the layer you are winding, don't worry, but keep winding as the coil is not that critical. Continue your windings from end mark to mark until you have used up the whole coil of wire. You should end up with a second 6 inch pig-tail of wire at the vise-jaws, which is your point of beginning, but many layers deeper. If you cannot make the last layer and have a 6 inch pig-tail, then unwind this layer back to the vise-jaws and measure out  6 inches from the jaws and cut off the excess, as one layer is not that critical.

Secure enough hard wood or plywood of 3/4-inch thickness to cut two blocks, 3 inches square. Bore a hole to fit the diameter of your bolt in the center of each block. Cut a third piece of wood 3 inches wide and 6 inches long. This will be the base.

Bore two holes 1/4 inch in diameter in one of the 3 inch square blocks. Bore the holes 1/4 inch from the edge of the block and 1 inch apart. Secure two bolts of 1/4 inch diameter and 1-1/2 inches in length with two nuts each bolt.

Now assemble the two blocks, one on each end of the bolt to the marks, which is now the ends of the coil. Then compress the end blocks to the coil ends and hold tightly by hand and nail on the bottom piece of wood to the end blocks. You now have the coil sandwiched between two blocks of wood which are nailed to a wood base. Now insert the 1/4 inch bolts through their holes in the one end block with the bolt heads facing the coil. Take the wire pig-tails, one to each bolt head, and make a turn or two under the head of the bolt and cut off the excess wire. Now put on one nut to the other end of each bolt and tighten securely as this is a permanent operation. Take the second nut, one to each bolt and you have a sandwich between the nuts for a cheap, but efficient terminal for your engine hook-up wires. You may now make one wrapping of the plastic electrical tape around the coil for protection against scarring the shellacked finish on the wire, but even this is not critical. You may now paint the whole coil and wood, for looks, as even liquid is not a critical factor to the coil.