Evac Horseshoe Magnet: How to Make a Strong Magnet

Evac, a super-charged horseshoe magnet to make your low-tension engine start like a high-tension one.

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by Dr. David Cave
Figure 19: The final product.

Learn how to make a strong magnet using Dr. Cave’s Evac horseshoe magnet design. Use your super-charged magnet make your low-tension engine start like a high-tension one.

The Evac methodology is not allowing the horseshoe conduit to touch the magneto pole pieces and surrounding the rare earth inserts with nonmagnetic material.

There have been at least three earlier attempts, probably more, to insert a rare earth magnet into low-tension horseshoe magnets. The thought was to use the huge increase in magnetism to produce a hotter spark, making an engine easier to start, and helping it run smoother and slower. In general, those early attempts resulted in limited success. Using a magnet that is many thousands times stronger, the magneto energy output increased only 10 to 30% at 500rpm and, at best, 5% at 50rpm where it’s needed. This article will point out the errors of earlier attempts and lay out the method that the Evac magnet uses to achieve the full potential of rare earth magnet inserts. Evac typically produces a 600 to 700% hotter spark. By following these steps, super strong magnets can be created for any low-tension magneto.

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Rare earth magnets are generally made of neodymium. Neodymium is one of 17 elements in a group chemists call the rare earths, thus, the term rare earth magnets. Readily available and reasonably priced, they come in a multitude of small sizes and shapes that are incredibly strong. A single 1-by-1-inch neodymium magnet that is only 3/8-inch thick will require more than 50 pounds of force to remove it from a steel surface.

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Principles of magnetism, and how to make a strong magnet

To successfully make a horseshoe rare earth super magnet, there are three simple principles that need to be understood. First, the magnetic field does not emanate from a north or south pole and fade off into space. Magnetic field lines always form a closed path or complete loop. Starting at any point on a magnetic line, say the north pole, tracing that line will eventually return to the starting point (see Figure 1).

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The second is that some materials are harder for a magnetic field to pass through than others. Those hard to travel through materials quickly reduce the magnetic strength as the distance traveled increases. A vacuum is most difficult for magnetism to pass through, but all the nonmagnetic materials (air, wood, plastic, aluminum, brass, epoxy, paint, etc.) are only slightly better. Ferromagnetic materials (those which stick to magnets) such as iron and its compounds (steel), cobalt and nickel, can be from 100 to 100,000 times easier for a magnetic field to pass through. Figure 2 is a horseshoe magnet with two possible paths for the magnetic field to travel from the north to the south pole as it forms a closed loop. The first path is directly through a 4-inch air gap (shown in blue). The second path is through a 0.1-inch air gap then through 100 inches of steel and finally through another 0.1-inch air gap to the south pole (shown in red). The magnet will find the 100 inches of steel and 0.2-inch of air easier than the 4-inch air gap and nearly 100% of the magnetic field will follow the easier, but longer, path. That principle will be used to route the magnetic field to where it is needed and to keep it away from where is not wanted.

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Finally, for a magneto to work, the magnetic field must pass through the armature core the coil is wound on. Figure 3 is a simplified cross section of the desired magnetic path, leaving the north pole traveling into the armature pole piece, across a typical 0.010-inch to 0.015-inch clearance (pole piece to armature) air gap, through the armature core, across a second 0.010- to 0.015-inch clearance air gap, through the south armature pole piece into the south pole, and finally up and around the horseshoe to complete the required closed loop. Although a magneto may appear to be made of pot metal or other nonmagnetic materials, two steel compound pole pieces are embedded in it.

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Using the above three rules to make a super magnet magneto means there must be a magnetic path from the inserted rare earth magnet north pole, through the north armature pole piece, across the air gap (between the pole piece and the armature), through the armature, across the second air gap, through the south pole piece, and up and around the horseshoe that is the absolute easiest path the rare earth magnet can possibly utilize. All other paths must have larger nonmagnetic material gaps. Put another way, the easiest path that an embedded rare earth magnet can take to complete its closed loop path is through the armature. Any other paths that exist must be more difficult, traveling through air or other nonmagnetic material. There are several ways to fail making that the easiest path.

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The earlier attempts to create a super magnet embedded the rare earth magnet in the horseshoe magnet. Figure 4 shows a cross section of these implementations. Figure 5 shows the magnetic short circuit created. Virtually no magnetism will follow the path through the armature with its two small air gaps. It’s difficult to see how the claimed 10 to 30% improvement was achieved using this approach.

The obvious solution is to put a nonmagnetic moat around the rare earth insert. If the moat is wider than the height of the inserted super magnet, the large nonmagnetic gap should present a much more difficult path than the desired path through the armature. Figure 6 depicts the rare earth magnet in one leg of the horseshoe with a moat. Figure 7 is a magnet that was fabricated with a plastic moat around the inserted rare earth magnet. Figure 8 shows a 1/4-inch bolt firmly held in the desired path, providing hope that this structure would be a success.

The magnet in Figure 8 was put on a magneto body and spun up. Zero output! Figure 9 shows a cross section of the new magnetic short circuit created. The closed loop field path that was easiest was passing from the rare earth north pole over to the armature pole piece, making a turn to travel in the pole piece beyond the magnet moat and into the horseshoe for an easy path to the south pole, thus completing the closed path.

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To successfully create a rare earth super horseshoe magnet magneto, there must be air or other nonmagnetic material between the horseshoe and the armature pole pieces and there must be air or other nonmagnetic material around the sides of the rare earth inserts. The horseshoe is a conduit for the magnetic path and need not be magnetized. The horseshoe is acting like the 100-inch of steel in Figure 2, and letting the magnetic field complete a closed loop. Any magnetism added to the horseshoe would be insignificant compared to the rare earth inserts. Cold-rolled or hot-rolled steel makes a fine horseshoe conduit. Figure 10 is a cross section of the final configuration showing the right leg of the horseshoe with the rare earth magnet insert and the corresponding armature pole piece. The nonmagnetic material is shown in green.

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Following is a step-by-step flow of the Evac magnet built for John Deere or Associated 2-bolt magnetos in a shop that did not have the capability to accurately bend 3/8-inch-thick cold-rolled steel. This first implementation placed two rare earth inserts in each leg of the horseshoe. Later examples were fabricated with one insert in each leg of the horseshoe. Other magneto brands will have different dimensions from John Deere’s but similar results will be achieved if the principles discussed above are followed.

Without the ability to accurately bend and form the horseshoe conduit, the structure uses two pieces of flat cold-rolled steel for the legs and a piece of thick-walled half pipe for the arch (see Figure 11).

Each leg is then machined to produce the recess for the rare earth magnets and clearance of the pole piece (see Figure 12).

After the legs are machined, the three pieces are welded (see Figure 13). The magnetic lines will not be disturbed at those two weld joints if there is a smooth fit with no air gaps.

The rare earth magnet inserts must be secured to the machined area of the horseshoe conduit. They tend to slide down then up as the horseshoe is pushed onto the magneto body. During operation, they tend to slide up until they reach the machined ledge and short out. A simple retainer holds the magnets in place nicely. The magnets do not need to fit tightly in the retainer. They are not going to fall out as they are held firmly by magnetic force. Retainer holes 0.020- to 0.040-inch oversize work well. Any nonmagnetic material can be used for a retainer: plastic, wood, aluminum, etc. Aluminum is readily available, durable, and easy to machine.

Several techniques were used to secure the retainers to the horseshoe legs. One method was nonmagnetic brass 8×32 screws (see Figure 14). The screws were placed to fall behind the magneto magnet retainer strap. Securing the retainers with epoxy or super glue works well. Epoxy must be used sparingly and with care not to get epoxy in the magnet hole. Let the epoxy set for 4 to 5 hours, then scrape the excess out of the magnet holes. Although epoxy has a cure time and must be clamped, it does eliminate the problems of bolt hole alignment of the two small holes on the magnet faces.

The thickness of the retainer must be the same as the rare earth magnet’s thickness. That must also be the exact depth of the notch in the horseshoe leg. More about that later.

The final assembly is ready for paint and rare earth magnets (Figure 15). The last step is insertion of the magnets. The author used DX03-N52 magnets from K&J Magnetics ($5.66 each plus shipping). These magnets are 1 inch in diameter and 3/16 inch thick, half the horseshoe conduit thickness. If the magnets are too thick, the remaining horseshoe material behind them gets too thin and is unable to transport the full magnetic field. On the other hand, thinner magnets have less strength.

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Caution:  These magnets are very strong. If one gets away and sticks to a metal surface it takes about 28 pounds of force to remove it. At 1-inch diameter and 3/16-inch thick, they are difficult to grasp and must be pushed over an edge to get them off. K&J ships them with plastic separators (Figure 16).

One leg of the horseshoe must have the inserted magnet’s north pole pointing out, the other leg must have the south pole pointing out. K&J Magnetics does not mark the north or south pole, so it is easy to get them in wrong. Mark one end of the stack (Figure 16) red and the other end black. When one magnet is taken off the stack, quickly mark the new end magnet with the same color as the one removed. When your Evac magnet is complete, one leg will have two magnets with red showing while the other leg will have two magnets with black showing. It isn’t important which leg of the horseshoe magnet is the north pole and which is the south pole, your engine will run the same either way. For more on this visit www.cave.engineering, click on Evac Associated and go to Basic Magneto and Igniter Operation.

Figure 17 is the first unit under test. At 50rpm cranking speed, the Evac magnet created 2.5x hotter spark than a run-of-the-mill John Deere magneto. At 400rpm, the Evac magnet created a 6.5x hotter spark. A unit built with only one rare earth insert in each leg is 1.8x hotter at 50rpm and 2.5x hotter at 500rpm. As a demonstration, a John Deere engine is regularly started by standing behind it, rolling the flywheel up on compression, then pulling with both hands on the top of the flywheels. It starts every time as if it were a high-tension engine (see Figure 18).

In Figure 19, the final magnets are visibly difficult to tell from original vintage magnets. J-B Weld was used to secure the retainers.

As stated earlier, high-performance super magnets can be produced for any low-tension rotary magneto by following the Evac magnet methodology. The Evac methodology is not allowing the horseshoe conduit to touch the magneto pole pieces and surrounding the rare earth inserts with nonmagnetic material.

Dimensions for the Iowa Dairy/John Deere/Associated 2-bolt/IHC Type R magnets are in Figure 20. The IHC Type R fits but is not the correct height. The most difficult task may be finding a piece of pipe with the proper ID and OD to cut for the arc. However, the horseshoe is a magnetic field conduit and only the ID is critical. The ID is critical because the completed magnet must fit snugly and squarely against the magneto body. Any misfit at the magneto body will create an air gap that is difficult to pass through in the critical desired magnetic path. The thickness of the pipe and legs must be the same, but not necessarily the same as the OEM magnet.

There is no physical reason that the top of the horseshoe needs to be an arch. Figure 21 is an Evac universal troubleshooting magnet. Being adjustable, it can be made to fit any horseshoe magnet magneto. The underside of the top piece and the top of the adjustable leg should not be painted to maintain good magnetic field flow.

Caution: By using larger rectangular rare earth magnets, the magnetic field could easily be doubled over that produced by the circular magnets used in this implementation. However, doing so may exceed the capability of the 24-gauge armature coil wire. The ease of starting a low-tension rotary magneto engine with the current 1-inch magnets didn’t warrant pushing the limits of 24-gauge armature wire further.

The magnet on your magneto can be viewed as a catalyst or facilitator. No energy in your spark comes from the magnet. The magnet simply provides a means for crankshaft energy to be converted to electrical energy. Getting a spark with 6 to 7 times more energy results in the magneto being 6 to 7 times harder to turn. Engines with external, well-worn magneto gears will likely experience increased noise from gear chatter.

Treat the rare earth magnets with respect. They can be dangerous and cause injury. Figure 22 is the result of a loosely held magnet jumping from the author’s fingers to another magnet a full foot away. The impact was strong enough to shatter both magnets. Screwdrivers, pliers and other tools tend to move around and become irritatingly magnetized.

After writing this paper, the author switched sourcing to totalElement as they sell a pack of five magnets for $20 with free shipping (part no. D1X316N52-5PK).

In addition to the Iowa Dairy, John Deere and Associated 2-bolt magneto dimensions, the author has established dimensions for IHC Type R, IHC Type D and Sumpter No. 12 magnetos.


A special thanks to engine enthusiast Doug Reynolds for CNC machining and welding our magnets.

Dr. David Cave is a regular contributor to Gas Engine Magazine and can be emailed at jdengines@cox.net

  • Updated on Sep 15, 2022
  • Originally Published on Sep 13, 2022
Tagged with: horseshoe magnet, how to, magnet, magneto, welding
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