John Rex explains some common misconceptions about magnetos and how they can lose their charge and offers advice for magneto maintenance.
A heavy duty 12 volt portable magneto charger made by the author (or use at local engine shows).
"How often do magnetos need recharging?" is a question frequently asked by antique engine enthusiasts. The answer has to do with the permanence of the magnet on the magneto. Magnets are mysterious to us because the magnetic field cannot be detected by any of the five senses, yet it does exist and has many powers. It can attract metal objects, convert mechanical energy to electrical energy, and vice versa, and even alter the normal characteristics of materials.
It is a common fallacy that a permanent magnet expends internal energy to create electricity from a magneto or generator. This fallacy leads to the belief that after repeated use the magnetism will all be used up and hence the magnet must be recharged. The energy to create the spark comes not from the magnet, but from the mechanical energy required to drive the magneto. The magnet merely acts to convert this mechanical energy to electrical energy.
Confidence in the permanency of permanent magnets is substantiated by evidence of the many applications they have been put to over the years. For example, magnets are present in compasses but they never wear out or need recharging. The continued accuracy of some of the most exacting scientific electrical measuring instruments such as the familiar speedometer also depends upon a permanent magnet remaining constant.
Most magnetos on hit and miss engines used magnets that were made of quench hardened steel alloyed with chrome, cobalt, or tungsten. Magnets of this material were made from rolled stock by forming or cutting into the shape of a horseshoe or long bar. Immediately after quench-hardening, the material was rather unstable metallurgically and considerable change in magnetism could occur if magnetized during this transition period. Fortunately most manufacturers allowed for proper aging after quenching and before magnetizing. There is probably some reduction in magnetism over the decades; however, it is minimal and probably only results in about one third loss at the most. Such a loss would hardly prevent a well-designed magneto from functioning adequately.
Owners of antique engines sometimes report that their magneto has mysteriously lost its magnetism. In most cases, the fact that the magnetism has been lost cannot be questioned; however, the 'mysterious' aspect can be disputed. There is nothing mysterious about it. If a piece of material can be magnetized, it is likewise capable of being demagnetized. There are definite and logical reasons why magnets lose their magnetism. The most common cause for these losses is that the magnets have been subjected to demagnetizing forces.
Investigation into how this happens usually uncovers at least one of the following:
1) Take the case of an engine enthusiast who wants to get the maximum performance out of his magneto. He removes the magnets, takes them to his local magneto service center for recharging, then puts them back on the mag. Now he gets a weaker spark than before. Why? Because removing the magnets subjects them to demagnetizing forces, the recharging restored the magnetism, but they again lost some of their magnetism upon removal from the charger. What's the right way to recharge them? Charge them on the magneto as an assembly. Removing the magnets from a magneto causes them to lose some of their magnetism.
It is possible to remove the magnets without weakening them by first installing a 'keeper' across the poles. The keeper should be a material which conducts magnetism well, such as soft iron. The keeper 'short circuits' the magnet and prevents charge loss during removal. One difficulty is that a keeper frequently must be a complex shape so the magnets can be removed while the keeper is across them. Magneto repair stations usually don't bother with keepers, because they recharge the magnets after all magneto work is completed (I usually totally demagnetize the magnets when working on magnetos so they won't pick up iron filings, magnetize tools, etc.)
2) Don't pile several magnetos together in a heap or pile. Having magnets close together can cause them to partially demagnetize each other. Keep magnetos away from each other by at least 3 inches.
3) Don't connect a battery to the terminals of a low tension magneto. The current can demagnetize the magnets and might burn out the coils. If you want to run a mag-equipped engine on a battery and coil, make sure the mag lead is disconnected so battery current will not demagnetize the magnets.
4) Don't remove the armature or rotor from a magneto because the magnets can be weakened. The rotor can be safely removed by first installing a 'keeper' across the end of the magnet poles. Leave the keeper in place until the rotor is replaced. The Wico EK mag is an exception to this rule and the armature may be safely removed and replaced without using a keeper.
5) Magnets which have been exposed to a fire or excessive heat will frequently lose their heat treatment and hence their magnetic characteristics. Such magnets may never recover their magnetism even when recharged because the characteristics of the metal have been altered.
6) Don't attempt to recharge magnets by methods which produce inadequate energy to fully recharge them. Wrapping a few turns of wire around the magnets and 'flashing ' the wire with an automotive battery or arc welder frequently exposes the magnets to demagnetizing forces and may weaken them rather than improve them. If recharging is required, it's best to use a magnet charger specifically designed for the purpose.
It is rumored that shock and vibration may demagnetize magnets, however I have never been able to observe this effect, even on early hardened magnet materials. I have seen magnetos that were dropped and the shock of the impact cracked the magnets, yet the mag still functioned, provided the magnets remained intact. In some cases after breakage the magnets will not stay in place. In cases like this, the magnets can be arc welded back together and still work adequately. Use a mild steel arc electrode with good penetration such as E6011 and place small tack welds at each end of the crack. Don't weld across the entire crack as too much heat will anneal excessive amounts of material and reduce the effectiveness of the magnet. Recharging after welding is required. Torch welding produces too much heat and should not be used.
Engines manufactured after World War II contained magnets that were usually made of Alnico (aluminum nickel cobalt) and are much more stable than quench-hardened steel alloy magnets. Special charging techniques must sometimes be used to adequately charge Alnico; however it is also much more resistant to demagnetization effects and almost never needs recharging. These modern magnets are usually short and small, internal to the magneto and are not visible from the outside. Alnico is not rolled from bar stock but is cast and has a rough surface texture except for the pole ends which are usually precision ground.
The answer to the question 'How often do magneto magnets need recharging?' depends on many factors. If the engine isn't running and all electrical and mechanical items have been tested and eliminated as possible sources of trouble, then the magnets have probably been exposed to demagnetizing forces as mentioned above and recharging is recommended. I would recharge quench-hardened magnets on general principles if a magnet charger is readily available. If you're going to the effort of having the magnets charged, make sure that all mechanical and electrical work, painting, etc. is finished before charging so that some charge won't be lost upon disassembly. If a magnet charger is not available and the magneto is working, I would use the rule which applies to all things and is, 'if it works, don't fix it!'
Stop by any major New Hampshire or Massachusetts engine show and chances are good that you'll find John Rex there offering magneto charging, magneto advice and limited magneto service all at no cost. John's interest in magnetos and old engines started about 5 years ago in 1981 after he found an old Fairbanks Morse 'Z' on a friend's Lake Champlain property. The engine, purchased about 1928, was used to supply electricity for pumping lake water. She never would sell John the engine, but this started his interest and he now has about 15 old engines and dozens of magnetos in his collection.
"One thing I noticed at engine shows was that no one seemed to have much knowledge about magnetos used on antique engines and if there were mag problems, people didn't know what to do." An electrical engineer by training, John decided to help people with mag problems. Last year (1985), he designed and built a heavy-duty portable magneto charger (it weighs over 150 lbs.) and started offering free charging at local engine shows where the service was an instant success.
"I was charging magnetos from early morning until after dark at this year's Orange, Mass. show. On Sunday, I finally had to say the heck with it and take an hour off just to see the show."
John occasionally does mag work at home, but since it's a hobby and not a profession, he limits his work to shows. The old mags and mag literature, much of which has been donated to his collection, provides a base for knowledge and a source of parts. He encourages people to do their own mag work and offers pointers on how to best accomplish it.
John is doing research on the Webster Hi Tension magneto and would like to correspond with anyone who has information on these or their history. He wrote this article to answer some of the frequently asked questions about magneto charging.