Gas Engine Magazine

Cheap Fix: Briggs and Stratton Coil Conversion

Briggs and Stratton ignition coil conversion saves money and headaches.

By Staff

This submission is not so much about a successful engine repair as it is about a clever and ingenious idea, which also saves a substantial sum of money in the event you’re a working stiff. I’m writing this for Kent David Redd, the creator of the idea, as he has an interest in passing it along. I’ve been told on a number of occasions through the years, “necessity is the mother of invention” and “work with what you have.” That’s how this all came about. I’ll start from the beginning, but I promise not to bore you to tears with details.

I have a Briggs and Stratton Model 14 engine that acted as if it had a partially plugged carburetor, since it would start and idle just fine. However, when attempting to throttle it up, the engine would die. After repeatedly pulling the carburetor apart and checking every conceivable possibility I could come up with, I gave in. Listing the engine for sale, cheap I might add, a couple fellas contacted me with an interest in the engine and asked me to describe the engine’s behavior, which I did.

I was consistently told the coil was shorting out at higher RPMs, but not at idle speed. So of course when I was turning the engine over, checking for spark, I assumed the coil was all right. I then decided if it was just a bad coil, I’d just keep the engine and replace it.

Six Months Later

After the engine sat in its dark corner for another six months, I decided I wanted to hear the baby beastie run once again. I knew I couldn’t afford a new replacement coil and the engine wasn’t worth the cost, so I posted an ad on the Internet looking for a functioning used coil but got no bites. The offers I did receive were for new “Made in China” or NOS coils, some at ridiculously high prices, while others were reasonably fair.

While considering the purchase of an NOS coil, Kent David Redd made me an offer for instructions … yes, illustrated instructions on how to convert a later model Briggs and Stratton coil to fit my Model 14. Person-ally, I thought it sounded a bit goofy, but it did spark my curiosity. After sending a reply to his offer and asking about the price tag, he forwarded said instructions via e-mail at no cost.

After reading through them several times and looking over the illustrations, the conversion began making sense. The end result was that I didn’t have to ask questions, the conversion only required approximately four hours of easy labor, not a dime from my pocket, and the Model 14 engine runs sweet as a dream. This is such an ingenious idea and I was so impressed with it, I talked to Kent about doing this story and he had no problem with it and liked the idea of getting this out for others to use. I’m very impressed with this, especially since at the time he came up with it, he, too, was between the proverbial “rock and a hard place,” so just maybe necessity is the mother of invention.

Creativity at Work

The coil to be modified needs to be, 1: From a Briggs and Stratton engine of 2-1/2 to 16 HP; 2: Points-type ignition (pre-1981), and; 3: Two-legged core rather than three. These coils can easily be found at a scrap yard from a trashed push mower.

The biggest key here is that the B and S Model 9, 14, 19 and 23 engines used what was referred to as “Magnamatic” ignition, which was very different than the typical arrangement in the other models. The main difference is that the coil was placed under the flywheel rather than on top of the outside of the flywheel, so space is pretty tight under there. Another difference is that the permanent magnets are not placed in the flywheel, so it needs no key and keyway for correct spark timing, but rather the magnets are placed in a “hub” of about 3-inches in diameter and mounted directly onto the crankshaft, this also being under the flywheel.

Removing the old coil from the Model 9, 14, 19 or 23 is self-explanatory. That coil sits on top of a laminated armature that surrounds the hub that the magnets are in. The concept here is to make the replacement coil fit where the other was – in a very tight space.

There are a few things I did a bit differently than Kent, so you will have a choice on how you want to do this. What is described is to remove the coil from the core laminations. I didn’t do that, but rather placed a small, 3/4-inch-thick block of wood under the core to proceed with the next step. From the top of the core (looking at the coil as if it were mounted above the flywheel), measure downward 2-1/4-inches and scribe a line across both legs. Using a milling machine (for best results), cut-off saw or hacksaw (preferably with fine-toothed blade such as 24 to 32 TPI), cut the bottom portion of the legs off at the scribed line. Accuracy is important due to the limited space.

If you do not remove the coil from the core, cut slowly so you don’t cut into the windings. It is suggested you place a block of wood on the underside of the coil and run a wood screw through the slotted mounting hole into the wood to keep the laminations from getting bent. You can also use a very small clamp to keep them together.

After those portions of the core legs are removed, the replacement coil will be close to 0.20-inch too wide to fit in the armature of the Magnamatic ignition. Therefore, 0.10-inch needs to be removed from each leg from the point of where the legs were just cut and 3/4-inch vertically.

The same block of wood can be used on the underside from the core – just use a disc grinder or file to gently remove the needed material. Be careful not to bend the laminations of the core, since they’re almost impossible to straighten. In the event you remove too much material, simply shim the required amount between the core and the armature.

The grounding lead of the modified coil can be connected to the same screw as the old magneto grounding lead. The primary lead going to the points only needs to have a ring terminal put on the end to connect it in the same manner as the primary lead of the old coil. The secondary lead (spark plug wire) will need to be lengthened via a splice in the shorter secondary lead of the modified coil. Solder the leads together, then, using silicone, make a very thick layer over the top of the splice to insulate it. A quicker method, if you don’t want to wait for the silicone to cure, is to use hotmelt glue. The job can be completed and ready for use in less than five minutes with this method.

Finishing Up

Securing the replacement coil cannot be done in the same method as the previous coil, so you will need to study your options at this point. In Kent’s first modification, wire was tightly fastened over top of the coil. The next step is to use a hose clamp, cut and modified in place of the wire. In my operation, I cut and added additional length to a hose clamp (you can simply buy a large diameter clamp if you wish, which is cheaper and easier) and placed the clamp over the top of the “new” coil and around the entire armature of the ignition. I don’t believe I have more than about 0.005-inch clearance between the band of the hose clamp and the inside surface of the flywheel at the point it runs over the top side of the coil – that’s just how tight things are under there.

From this point comes the gravy work: Reassemble the engine components and start your beastie up … it’s a real thrill after having previously given up on it! Put this idea in your memory bank, because even if you can’t use it now, you never know when you might.

  • Published on Nov 1, 2005
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