Servicing Magnetos

By Staff
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Plate No. 8: Showing the spark gap setting for spark rack on Maeklim Magneto test benches.
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Plate No. 1: Showing how to charge bar magnets on Makelim Heavy Duty Magnet Charger.
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Plate No. 2: Show how to charge rotating Alnico magnets through frame of magneto on Makelim Heavy Duty Magnet Charger.  
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Plate No. 4: Show to charge flywheel magnets on Makelim Heavy Duty Magnet Charger.
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Plate No. 3: Showing how to charge horseshoe magnets assembled on magneto on Makelim Heavy Duty Magnet Charger.
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Plate No. 6 Showing method of charging four pole Alnico Magnets on Makelim Heavy Duty Magnet Charger.
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Plate No. 7: Showing difference in construction of present-day magnetos compared with old-style four magnet type. Small magnetos have Alnico magnets. This type of inductor magneto was widely used 25 years ago before discovery of Alnico magnets. Note the four horseshoe type magnets and impulse starter.
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Plate No. 5 Showing how to charge Alnico Rotating Magnet on Makelim Heavy Duty Magnet Charger.

The following article was sent in by Craig Roy of 1513 Beach St., Salina, KS. 67401. It was originally published in the H.G. Makelim Co. 1957 catalog, and is being reprinted here with permission from Chuck Hess, whose father bought the company in the late 1950s. Unfortunately, the company stopped production of magneto test equipment in 1977.

A brief history
In the days of the Model T Ford practically all automobiles used magneto ignition. Some, like the Ford, used what was known as low tension magnetos. Low tension magnetos practically passed out with the passing of the model T. However, Scintilla Magneto Company is again making low tension magnetos that are being used with success on some types of heavy duty stationary and marine engines where extra long spark plug cables are necessary. This is due to the fact that much of the heat value of the current in a high tension magneto is dissipated in the long spark plug cables.

The writer learned magneto repairing in the days when such swanky cars as Stutz, Lozier, Mercedes, Winton, S.G.V. and many others used magneto ignition as standard equipment. In those days magneto service stations were as numerous as battery service stations are today. Cars came without starting or lighting equipment and the hand crank was used to start the car. When storage batteries came on automobiles it was found that the battery could be used for starting and lighting and it looked like the magneto business was doomed. Slowly but surely one automobile manufacturer after another discarded the magneto and used battery ignition. Batteries furnished light, ignition, and power to start the car whereas the magneto furnished only ignition. Many service shops sensing the end of magnetos gradually discontinued magneto service and went into the battery business. However, there were many places where batteries did not prove as efficient and reliable as magnetos. Farm tractors were coming on the market in great numbers and the storage battery did not stand the rough treatment and neglect generally given by a tractor operator. Plowing rough fields would crack battery jars, batteries would run down, plates corrode when not in use and they had to be constantly recharged, etc., so magnetos began staging a comeback on tractors, industrial equipment, outboard motors, concrete mixers, fire engines, racing cars, airplanes, Army tanks, etc., where a reliable self contained ignition system was essential. What was considered a dying business twenty-five years ago is now a very live and growing industry.  

Today there are seven or more prominent magneto manufacturers and one of these has built three million magnetos in the last ten years. Another manufacturer has built in the neighborhood of one million flywheel magnetos. There is a widespread use for flywheel magnetos on lawn mowers, scooters, wood saws, washing machines, wheelbarrows, small compressors and lighting units, etc. You can readily see that there is going to be a lot of magnetos in need of service for a long time to come.

Modern magnetos
There are three different types of magnetos in use today, namely: Shuttle wound types, inductor types, flywheel types.

In all shuttle wound types the high-tension coil or winding spins around with the armature while the magnets remain stationary.

There are are two kinds of inductor type magnetos. In some the magnet revolves on the shaft and in others a rotor revolves while the magnet is stationary. In both types the winding or coil is stationary. The cost of building an inductor type magneto is considerably less than the shuttle wound type. As a general rule the inductor type magneto has fewer parts and requires less time to repair.

Flywheel type magnetos are also an inductor type. The magnet revolves and the coil is stationary.

Impulse Couplings: Many magnetos are equipped today with impulse couplings to make for easier starting and less cranking. It is a mechanical device that is installed between the magneto and the engine drive. Its purpose is to intensify the ignition spark at low rotative speeds. It also provides the means of automatically retarding the ignition spark during the starting period thus reducing the possibility of damage to the engine or injury to the operator due to back firing. It generally consists of a shell and a hub connected together by a strong spring. One half of the coupling is fitted to a drive member of the engine shaft while the other half is keyed to the magneto rotor shaft.

Repairing and testing
A clean bench and a clean shop are the first essentials of a magneto service business. Extreme care should be taken to see that work benches are wiped clean of dirt, grease or any metal particles that the magnets or magnetic rotor will pick up. We have seen great damage done to a repaired magneto that had picked up a minute particle of wire from a buffing wheel in the course of repair.

The first step in repair of a magneto should be a thorough cleaning of its exterior. After cleaning, the magneto should be placed in the vise and turned over slowly by hand. If there is a binding or rubbing action, no further steps should be taken before dismantling, since such a condition indicates worn bearings or some internal trouble. The pull due to magnetic break should not be confused with binding.

If the rotor turns freely (except for magnetic pull) the magneto should be run slowly on the Test Stand with wires connected to distributor block. At slow speed note whether or not impulse coupling is working properly. Watch the spark gaps to see if all are firing at slow speeds. Gaps should be set 5 mm. distance apart. If set too wide magneto will fire in safety gap. Static gap should be set 0.005 from insulated electrode. (See Plate No. 8) If no spark appears on one or more of the spark plugs, this should be noted on work card, so condition can be corrected. Increase speed of magneto and observe spark rack to see if spark is cutting out at high speed. Check ground switch to see if it is working. If these tests indicate trouble internally, the magneto needs dismantling.

Remove magneto from Test Stand and dismantle. Remove Impulse Coupling.

Check distributor block for sticking, worn or corroded brushes or carbon tracks. Check breaker contacts for wear, misalignment or oxidation. Check breaker cam for excessive wear. Check distributor rotor or collector ring for leaks, wear and carbon tracks.

Remove high tension coil and test on winding tester. A commercial battery coil tester does not give a satisfactory test on magneto winding unless provision has been made for inserting a series resistor in the primary circuit. Use one that will show the current draw of the winding sufficient to jump a 5 mm. gap.

Remove condensor and test for leakage, capacity and resistance. Check timing gears. Note markings on gears before removing.

Remove magnetic rotor and check bearings carefully. If worn or pitted replace complete bearing. Do not replace only half of the bearing or separate balls. See that magnetic rotor is tight on shaft. Check bearing insulators. Check impluse coupling. Watch for wear on drive lugs, coupling shell and pawls.

With all these parts in a pan after testing, proceed to wash thoroughly in a wash rack containing either white gasoline or solvent. Parts that have been painted, like housings, etc., are either buffed on a wire wheel to remove paint or dipped in a solution that will remove paint without damaging the castings.

We endeavor to repaint with colors as used by the original magneto manufacturer.

The magneto is now ready to be reassembled. See that bearings are well packed with high melting point grease as recommended by the magneto manufacturer. Do not use oil. See that bearings are properly shimmed. Not too tight or too loose. This is important. See that contact points are clean and properly aligned. Set points as specified by magneto manufacturer. If pitted, a small fine stone can be used for resurfacing. If badly pitted it is better to replace. Make sure no oil or grease gets on surface of contacts. Replace broken or worn gaskets.

Testing lag angle of impulse coupling
On all our Test Benches we have a degree dial or protractor. Take one wire from distributor cap and fasten to terminal on degree dial. Start motor and revolve magneto in direction or rotation until impulse pawls disengage from impulse catch plate or stop pin. A spark will occur on degree dial from pointer. For convenience in reading, if spark occurs at bottom of degree dial you can use a different wire from distributor cap in order that spark occurs at top of dial where it is easily seen.

Revolve dial until spark occurs at zero reading on scale and stop motor. Then turn bottom drive of Test Stand by hand in the direction that will revolve the magneto in the proper rotation and note where spark occurs past zero mark. This will indicate the lag angle of the impulse coupling. Do not take this reading while impulse is operating under motor power as it will not be correct. Care should be taken to ascertain what degree of lag angle is required on the engine the magneto is installed on. Different engines require different degrees and they vary from 10 to 55 degrees.

When damaged or defective parts have been replaced and magneto has been assembled and recharged, proceed to test at slow and high speed on Test Stand. We give a running test of ten minutes.

Spark gap setting
In order to protect the coil or winding and other current carrying parts a Safety Gap is provided in a magneto. If for any reason the electrical resistance of the secondary circuit is increased to a high point as when a spark plug cable is disconnected, spark plug gaps too wide, or the gaps on Test Stand Spark Rack are set too wide the high tension current will discharge through the safety gap. The current should never be allowed to jump across the safety gap for any length of time. Jumping the safety gap can be detected by a snapping sound and an immediate search should be made to detect the trouble. Care should be taken to keep spark gap points clean and free from oil, dirt and moisture. They should be kept sharp and evenly adjusted. See Plate No. 8.

If magneto tests o.k. proceed to bill from work card all parts replaced and add labor.

The successful service station owes its success to a clean, well equipped shop and expert mechanics. Dirt, oil, grease and grime are the greatest factors in magneto breakdowns. A clean, orderly shop promotes good work and results in customer confidence. Keep clean.

Method of changing magnets
Magnets must always be placed on the charger with the north pole of the magnet against the south pole of the charger. Two methods can be used to determine the correct position of the magnet in relation to the charger. One good method for horse shoe type magnets is to suspend the magnet above the charger while the current is on in the charger and allow the magnet to assume its own position. The magnet can be held lightly between two fingers at right angles above the charger.

The magnet will turn and line up with the charger poles in the right position. To do this it is only necessary to close the switch on the charger for two seconds. Leaving the switch on for longer periods overheats the coils on the charger and can damage the charger coils. Repeat this operation three times, opening the switch slowly to draw out a decided arc.

We use a knife switch on our chargers. A solenoid or starter switch is sometimes used and there is no arc that you can see. However the arc is there, inside the switch and in trials in our shops we have had the switch contacts fuse and stick and burn up the charger coils.

To charge rotating Alnico magnets
Various magneto manufacturers have different methods of charging magnetic rotors. We find that by placing the assembled magneto (less impulse coupling) in the field of the magnet charger, the magnets can be charged to factory specifications.

See Plates Nos. 1, 2, 3, 4, 5 and 6, showing various methods of charging different models of magnetos.

Care should be taken not to reverse the polarity. Do not charge magnets with impulse coupling on magneto as you may charge the hardened impulse parts so they will not engage.  

Note:Charging Blocks No. 130 are used in this operation.  

Note: special Charging Blocks No. 131.  

Determining the north from the south pole
When a north and a south end of two magnets are held close to each other they will pull together or attract each other. If two north or two south ends of magnets are held close to each other they will repel each other or pull apart.

Use of magnet keepers
A keeper is a piece of metal placed across the poles of the magnet to retain its magnetism. Many magnetos in use today can be charged and assembled in the magneto and a keeper is not necessary.

Magnetos using horse shoe magnets can be charged as per Plate No. 3. Magnetos using Alnico rotating magnets can be charged as per Plate No. 5. Flywheel magnets are generally charged with the aid of various blocks that make a connection between the magnet poles and the charger poles. The charging operation should only consume a very few seconds. Leaving the charger switch on for longer periods may damage the charger coils.

Prices for charging magnets vary from 50 cents to $1.00.

Magnet charging is an essential part of a complete magneto overhaul.

Service equipment needed to service magnetos
The equipment required to service various makes of magnetos varies according to the volume of repair work done. If the volume is large we recommend our Model A magneto Test Stand capable of driving magnetos fitted with impulse coupling at a speed of 2500 R.P.M. This stand has a variable speed reversible motor so magnetos can be tested either clockwise or anti-clockwise. It has a degree dial divided 0.360 scale which will determine lag angle of impulse coupling. A speed indicator is desirable but not always necessary. Our Model A stand is equipped with six adjustable three point spark gaps.

A heavy duty magnet charger capable of completely saturating both Alnico and horseshoe type magnets.

A magneto winding tester. We are now building our own Magneto Winding Tester. It will test complete magneto armature with or without condensor, single or two spark windings, flywheel magneto windings. It will locate defects in surface insulators such as collector rings, conductor bars, distributor blocks, etc. A special chart showing whether or not a winding meets factory specifications and the maximum current required to fire a 5 m.m. open gap is given on the majority of various makes of magneto windings. You do not have to guess. A winding tester is of little value without a chart showing the current draw of the primary of the winding sufficient to jump a 5 m.m. open air gap.

A condensor tester that will test capacity, leakage and resistance. There are several good makes on the market.

An armature growler for demagnetizing impulse coupling parts, should they become magnetized.

A small bench lathe for turning armatures facing distributors, discs, etc., is very desirable.

Special magneto bearing pullers. Our pullers No. 108 and No. 109 handle the widely used E 15 bearing. Bearings are expensive and can seldom be removed without damage with special pullers.

Impulse coupling puller. Bench vise. Arbor press for pressing bearings on armature rotor, etc. Magnetic output meter for determining proper saturation for magnets. Soldering iron, screwdrivers, soft hammers, small wrenches, hand tools, etc.

A magnet charger is an absolute necessity in a shop repairing magnetos. A magnet is not like a storage battery. A storage battery must constantly be replenished with electric energy or it will become discharged. When a storage battery becomes discharged due to a short in the wiring, generator not charging, or excessive use of lights, radios, etc., a recharge will bring back its energy.

A magnet can be charged to its capacity and set on a shelf with a keeper and it will never lose its magnetism. It can be installed in a magneto and it will retain its magnetic strength until something happens that causes its discharge. Some people are of the impression that when a magneto fails to function all that is necessary to restore its usefulness is to recharge the magnets.

In most instances this is not the case. Something has caused it to lose its magnetic strength. This can be traced to one of several things. Broken or loose bearings allowing the magnetic rotor to drag against the housing, dismantling the magneto and removing the magnetic rotor or coil, warped housing causing the magnetic rotor to rub, and heavy vibration, etc., all can cause loss of magnetic strength. When this condition occurs, the magneto should be dismantled and the cause of the trouble located and corrected. It should then be given a recharge on a Magnet Charger built to thoroughly saturate the magnets.

Many of the old style Magnet Chargers will not thoroughly saturate the type of magnets in use today. Some of the old style magnetos had as many as four double bar horseshoe type magnets. These magnets were made of chromium and tungsten metals whereas today the majority of magnetos use Cobalt or Alnico steel for their magnets.

The following chart shows the force necessary to saturate the various grades of material:



Ampere turns/inch

Chromium and Tungsten



Cobalt grades



Alnico I, II, III



Alnico IV, V



The development of aluminum, nickel, cobalt alloy and steel known as Alnico has practically obsoleted all other steels as permanent magnet material, at least in the magneto field. Alnico has more available energy for a given volume than any other magnetic material yet discovered. Because of its higher coercive force, smaller and more compact magnets may be used, permitting considerable improvement in the design and construction of magnetos.

Magnet steel such as tungsten, chromium and cobalt can be cast, forged or rolled into the physical shapes desired and can be machined for the final fit. Alnico steel can be either cast or ‘sintered,’ a process by which the powdered metals of the alloy are formed into the desired shapes by the application of heat and tremendous pressure. Alnico is an extremely hard crystalline metal and cannot be machined by ordinary methods. It can however be cut with a grinder or a rubber wheel.

Note: The four horseshoe type magnets and impulse starter.

The service information given here is for the man who is just starting to do magneto repairing. We do not tell how to charge or repair any particular make of magneto but we have tried to give the main fundamentals of charging and repairing various makes. We do not assume that this information will tell a magneto repair man all he needs to know. We know of no book that will. Experience is the best teacher.

We do hope and believe that the information given here, gleaned by us in over thirty years of magneto servicing will be a help to many beginners in the magneto repair business.

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