Magneto Identification and Testing

Low tension or high tension? Base or flange mounted? Learn the ins and outs of magneto identification

| March/April 1995

Webster tri-polar – low-tension magnetos.

Webster tri-polar – low-tension magnetos.

Photo by Bud Motry

Magnetos are classified as either low tension or high tension, depending on the application. However, a low tension magneto was of simple construction. It used a coil of a single winding, did not require a more complicated coil of both a primary and secondary winding, nor a condenser, nor contact points. It was merely a dynamo, or in other words, an electric device that created a current flow. A primary wire connected to a breaker point, called an igniter, created the spark when the flow of current was interrupted. As compression and speed increased, the low tension magneto was not sufficient to generate the spark needed for ignition Therefore, the high tension magneto took over the field of ignition.

Magnetos are further identified as BASE mounted or FLANGE mounted. The old John Deere "D" used a base mounted magneto. Later models all used a flange mount. The direction of rotation was either clockwise or counter-clockwise, sometimes referred to as anti-clockwise. The direction of rotation was determined as viewed from the drive end. Common magnetos used either a LUG drive or a GEAR drive. Most tractors used a lug drive. Many gasoline engines used a gear drive. In case of the John Deere tractor, the lug drive was either a short lug or a long lug. The long lug extended approximately 1-1/8-inch to 1-inch from the flange mount to the end of the lug. The short lug extended approximately to 13/16-inch from the flange mount to the end lug. I would suggest this clearance be checked whenever changing magnetos.

Most magnetos used an impulse coupling with the drive, to provide a good spark even with a slow turning movement, such as a hand crank. Incidentally, for the novice, never spin a tractor engine with a crank. Use a quarter turn of the crank only. Pull upward, with all five fingers on the same side of the crank handle. I have seen old timers crank a tractor engine by placing the crank handle in a position just to the right of top center, and then holding onto the radiator, place a foot on the crank and give it a hefty shove. This is a quarter turn that was suggested.

Some gasoline engines such as some models of the Fairbanks-Morse engine did not use an impulse coupling. It was necessary to spin the flywheel to get a good spark. One magneto of this type was an FM type "R." I never did have much use for this magneto and would replace it with an FM model J1A2. I recognize the fact that some enthusiast would not settle for anything but the original. That's okay, too!

I have an intense dislike for most cranks, especially the one used on the Fairbanks Morse Z style "D" engine. As an alternative to a crank, I would suggest filing a notch on the pulley to make a rope start device for the engine. Tie a knot on the rope and place it in the notch and wrap the rope around the pulley three or four times. The only safe cranks to use on an engine are the types pinned to the flywheel.

The John Deere "E" and the International "LB" are examples. Most engines can be started by turning the flywheel by hand. This includes all of the engines using the WICO model "EK" magnetos. On this magneto, keep in mind that there must be a spring action pulling the armature from the core poles. Just pulling the armature to separate it from the magnetic poles without the spring action would not produce a good spark. When starting an engine by turning the flywheel by hand, the direction of rotation is important. On most engines of the two-flywheel design, the direction of rotation of the flywheels was so called, right-hand rotation. This direction was accomplished, on horizontal engines, by standing at the rear of the engine-opposite the cylinder head and valve mechanismplace your hands on top of the flywheels and pull toward you.

Mike Henry
3/26/2013 1:02:33 PM

To test the output on a low-tension mag, I have always used a neon light bulb to test the output voltage. A neon lamp will flash with a minimum of 70 volts.