Dismantling Antique Tractor Magnetos

A guide to non-destructive dismantling and magneto restoration from a master of old tractor electronics

How to Restore Tractor Magnetos by Neil Yerigan

Keep your collectable vintage tractor electrical system up and running with restoration advice (and plenty of wisdom) from expert Neil Yerigan in his now back-in-print book “How to Restore Tractor Magnetos.”

Cover Courtesy Octane Press

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How to Restore Tractor Magnetos (Octane Press, 2011) is the essential guide to farm tractor electronics and offers the tractor restorer all the information needed to restore, repair and diagnose magnetos. Authored by the late Neil Yerigan, who was a master of working with vintage electronics, the book comprehensively covers how magnetos function as well as how to troubleshoot and repair common problems. In the following excerpt, Yerigan explains how to dismantle antique tractor magnetos non-destructively during the restoration process. 

Buy this book from the Farm Collector store: How to Restore Tractor Magnetos.

As there is more than one way to skin a cat, there is more than one way to dismantle an antique tractor magneto. I prefer the non-destructive system. The idea is that if you don’t break it, you don’t have to fix it. There are four requirements for non-destructive dismantling: time, patience, the proper tools, and good technique. If you have a Type A personality—aggressive, hard driving, impatient, a perfectionist—hire someone to do the work. If you are a Type B, or just naturally mellow, continue.

The nemesis of the repairer is the common machine screw. If you can master the removal of stubborn, rusty-headed, narrow-slotted screws, restoration is a piece of cake. If you lack the patience to do the job right, you are doomed.

The engineers who designed those tractor magnetos that we consider to be antique (or at least on the old side) really did a fine job. They used the best materials and technology available at the time. While some modern devices are designed to be thrown away, old machines are designed to be serviced or repaired. Of course, the engineers did not anticipate that, fifty or seventy years after the fact, some fool would try to resurrect and restore a magneto that had been abandoned in the dank corner of an unheated, dirt-floored pole barn since heaven knows when.

The main problem is that the tools that must be used to loosen the old rusty nuts, bolts, and screws just aren’t strong enough to do the job. Often, the fastener itself will break, leaving you with the problem of drilling it out and threading the hole. The only thing you can do is to patiently take the time and use the best tool for the job. Use a carefully honed technique, and hope for the best.

Some Non-Destructive Dismantling Techniques 

The first thing to do is to stop for a minute or two and look at the magneto that you wish to dismantle. Clean off most of the grease, dirt, or loose paint and shake out the leftover wasp nests so that you can see how the magneto is put together. Next, gather the tools that you think you will need. Make sure that your screwdriver blades are the right size. If your memory has started to slip, a notebook and pencil may be helpful. On magnetos with horseshoe magnets, I like to make a note about which side the north pole is on. International Harvester frequently stamps an N on the north pole side (a small pocket compass comes in handy). If you forget to do this, don’t worry, you can always reverse the magnet if the magneto doesn’t perform up to snuff. Most of the time, the orientation of the magnet doesn’t make a difference.

Some people like to use a muffin tin when they dismantle a magneto. They place screws and small parts in order in the muffin tin cups. When the time comes to reassemble the magneto, they install parts in reverse order. If they don’t have any parts left over when they finish, they can be sure that they did the job right.

Magneto Restoration: Where to Begin 

Common sense tells us to remove the most fragile parts first, and place them where they won’t get broken. Having removed the cap and rotor and maybe a coil cover, the next thing to do is to decide which end to start dismantling from. My preference is to start at the driven end and remove the impulse or the drive member. Except for some motorcycle magnetos, magnetos generally have a woodruff key to keep the drive components aligned. Always remove this key before beginning to take the other end apart. If you don’t get rid of the key, as surely as the sun will rise tomorrow, that rotor will slide forward and the key will slice a hole through the seal. It’s no big deal. However, a replacement seal is hard to find for some older magnetos. The outer race in the driven end bearing frequently holds the oil seal in place. The driven end bearing is usually the most difficult to reach. If the seal is okay and the outer bearing race is in good shape, it is best to leave well enough alone (if you are going to dunk the whole housing in paint remover or carburetor cleaner it is best to remove both the seal and the outer bearing race first, anyway).

Most of the time, I just try to observe the type screw head and length and number that are used, since I usually toss all the parts into a pan containing solvent. Using a small strainer with the solvent pan works nicely. Place the strainer in the pan, and drop the screws as you remove them into the strainer. You can swish them around a bit to clean off the grease and then lay them in a logical order on a paper towel.

I just sort the screws into groups by size, shape and length. Most of the time you find that the plate with four holes will match up with a batch of four screws, and a plate or assembly held on with three screws will match up with three similar screws. Of course, when I am dismantling something I make mental or written notes of things that vary from the norm.

For example, the housing which contains the points and condenser in an International Harvester F series magneto is held in place by two screws and a bolt with an extension on top.

The extension holds the metal lever that in turn holds the point cover in place. The two screws are of different length. If you make a mental note that the three screws are different, it is easy to remember what goes where.

Splitting Parts 

Along with the worn out screwdrivers, broken knife blades also come in handy for separating aluminum parts. Magnetos that use the three-piece ball bearings usually don’t use gaskets between the two major parts of the magneto. Those old-timers did some very precise machining and matching of parts. Frequently, you need to tap a blade into the almost invisible joint between two parts in order to wedge them apart. The aluminum is soft and if you start to pry around with a screwdriver you will surely leave signs behind. Horseshoe magnets that have been in place for sixty or seventy years also require a little persuasion to start moving. Once they have broken loose, wiggling and tapping are usually enough to remove them.

Shuttle-wound coils have brass end pieces that bend and distort if handled roughly. They also have a collector ring made, in most cases, of Bakelite (Bakelite, which is still used today, was the first real plastic to be put to common use). These collector rings are not extremely fragile, but if handled roughly will break. To dismantle the armature assembly, first you must remove the inner race of the bearing next to the collector ring. I usually use a special tool for that. I use the bearing separator and press it off using the hydraulic press. You can also use a two-jawed puller instead of the press.

Once rid of the inner race and its spacers, remove the metal shield next to the collector ring. It will slip off easily. Usually the collector ring just slides off. Of course, after being in place for a long time, you may have to use force.

I have yet to devise a tool that will hold the ring without breaking it. So I either curl my left thumb and index finger around the center of he ring and smack the end of the shaft with my brass hammer, or I use an underhand hold and slip the ring between my index and my middle finger and again use the hammer. Most of the time it works. If you do break a chunk out of the ring, don’t panic. Super Glue will come to the rescue. Don’t forget to pick up all of the pieces.

I worked on an old Wico magneto that had the slip ring made in three parts, and it screwed on to the armature shaft. I had banged away with the brass hammer, but the slip ring didn’t want to come loose. I used the magnifier and discovered something that looked like threads. Joe Malan at Standard Magneto confirmed that the collector ring screwed on. I couldn’t unscrew it until I accidentally managed to knock a chip out of the ring. Then I tapped it around with a small hammer. It pays to examine the collector rings carefully.

How to Fix a Tractor’s Broken Screws 

The key to drilling out a small broken screw is to drill a small pilot hole exactly in the center of the screw. Over the years I have found one sure way to do the job. First use the Dremel MotoTool with the small fluted ball cutter to remove the rough edge of the break. I particularly like the round ball because it enables you to do the job freehand. A flat-end mill of the proper size does a good job, but everything has to be perfectly aligned in a drill press or a milling machine.

You can wiggle the round ball until you have a good depression in the center of the broken screw. By removing some material around the edge of the screw, you can see exactly where the edge is. Next, take a needle point punch and tap it into the very center of the screw. I always double check, using the lighted magnifier. If the dimple is right in the center of the screw, I use a slightly larger and blunter punch to enlarge the dent so that a very small drill will be guided to the center. If the small dimple is a little to one side, slant the slightly blunter punch so that the dimple is moved toward the center. When you are convinced that the mark is absolutely on center, use the ball cutter to remove the little ridge pushed up by the punch. This is so that the drill will not be pushed off center.

Mount the magneto housing on the drill press, so that it is absolutely level. I usually start with a 1/8in drill bit. Watch when the drill first contacts the broken screw. If it starts to move to one side, carefully align the housing (or whatever you are drilling) so that the drill bit doesn’t move off center. Add a drop of cutting oil and drill the hole all the way through the screw. When the first hole is complete, carefully examine to ensure that the hole is exactly in the center. If it is, enlarge the hole with a second drill bit a little larger than the first. Check for center again. If it appears to be perfectly centered, select the drill bit that you would use to drill a hole to be tapped to the size and thread of the broken screw.

Drill the final hole. When you are done, you probably can see the edge of the original threads.

Use a sharp pick to peel the coil of material that made up the threads of the broken screw. When you do this, the person watching over your shoulder will be impressed. Run the proper tap through the hole to clean up the threads. If, however, after the first or second drilling you decide that the hole is a little off center, select a drill bit smaller than the one used to drill to tap size. Drill using that small bit, lightly taking material off of one side of the hole until you see the edge of the threads. Take a sharp punch and bend the thin edges of the old screw toward the center. Usually, you can shake the screw out or grab an edge with a needle nose pliers and wiggle it out.

If you completely screw up, and have the drill remove all of the thread material on one side, you have two choices. Depending upon the location, and the function of the screw, you can drill out to the next screw size. The new screw will not be properly centered, but it may still work. The second choice is to use something like Loctite Thread Restorer. This is an epoxy formulation that you use to fill the hole before inserting a screw that has been coated with a “release” material. The release material comes as part of the kit. After the epoxy has hardened, you remove the screw leaving a perfect threaded hole behind. I’ve used this product and it worked well.

Removing Broken Bolts 

For removing larger broken bolts or studs I follow the recommendations of my good friend Frank Bethard. Frank uses left-handed drill bits (Snap-on Tools and other tool companies have them). First he gets a good dimple dead center. Running the drill press backward and quite slowly, he uses greater than normal downward pressure. If the drill bites tightly enough, it will back the broken screw out. If it doesn’t do the job, you haven’t lost any time. For larger bolts, Frank recommends using a drill bushing to center and steady the drill. If the bolt has broken off below the surface of the part, use a bushing with a male thread and screw it into the hole. The bushing will guide the drill all the way through the broken bolt. It eliminates the need to smooth out the broken surface. If the left-handed drill bit doesn’t loosen the bolt, you are in good shape to try a straight flute screw extractor (it helps to select a bit of proper size to use a given extractor). Be careful when using screw extractors. A broken screw extractor is very hard to remove. They are too hard to drill: removing one may require drastic and time consuming effort.

If part of the broken bolt is protruding above the surface, a female threaded bushing may be used to make a guide hole for drilling out the bolt or for installing a straight fluted extractor.

I almost never use extractors, since the screws and bolts used in magnetos are fairly small and don’t lend themselves to extractors. My friend Frank does a lot of automobile restorations where larger bolts are used. He is adamant about not using left-handed spiral-tapered extractors. They exert pressure outward against the sides of the stuck bolt which doesn’t help in removal.

Most of my set of left-handed spiral-tapered extractors have become permanent residents of the junk pile.

This excerpt has been reprinted with permission from How to Restore Tractor Magnetos by Neil Yerigan and published by Octane Press, 2011. Buy this book from our store: How to Restore Tractor Magnetos.