Low Tension Magneto

The basics of how low tension magnetos work

Fig. 2

Fig. 2 - The springs forcing the lever to rebound beyond the at rest position of the lever.

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There are several types of  low tension magnetos that are very popular in use on antique engines. Webster, Sumter, and Wizard were a few of the manufacturers of the oscillating type, identified by the extension springs on the front of the magneto as shown in the photo.

These magnetos were operated by a push rod that moved a lever to produce a movement of the armature around an arc of approximately 30 degrees. Another type of low tension magneto was the rotating armature type, very commonly used. We will discuss the oscillating type at this time, and will explain the rotary type in a subsequent article.

A low tension magneto had only one winding that was needed to produce a flow of current. It is an alternating current but this is of very little significance.

Basic electricity study tells us that all we need to do to produce an electric current is to move a steel bar through the magnetic field between the poles of a permanent magnet. The faster you move the steel bar and the greater the magnetism, the greater the current flow. This is the basic principle of a dynamo, generator or magneto. The current flow does not produce the voltage necessary to create a spark that we can see or feel, as you can with a high tension spark.

The low tension magnetos were usually used with an interrupter or spark igniter, rather than a spark plug. However, with the use of an external coil and interrupter, the low tension magneto could produce a high tension spark to be used with a spark plug. Henry Ford utilized this method on his model T engines.

The principle involved with the igniter was that if you break a connection while current is flowing through a wire, you produce a spark. You may have noticed this when you pull the plug of an electrical appliance or drop cord. The flow of current from a low tension magneto is of extremely short duration when it is at its peak. At this time while current is flowing through the contact points of the igniter, they must be snapped open, very rapidly. This is accomplished through a system of levers and rotating shafts that will be described in more detail later (see figures 1,2, and 3).

This type of magneto was mounted on the igniter bracket. The igniter was bolted to the engine. The only timing required was to adjust the movement of the trip lever on the push rod. The magneto shaft lever must be tripped when the piston was at T.D.C. on the compression stroke.

In order to check the magneto and igniter, remove them as a unit from the engine. Place them in a vise or steady position. Now take a bar or screwdriver and place it over the end nut of the front end of the magneto shaft (see figure 5) and under the pin at the inner end of the spring as shown. Push down on the screwdriver handle to extend the tension of the springs. Pull the handle towards you to release spring tension.

At this time you should be looking at the contact points. When the contact points snap open you should see a nice fat blue or greenish spark. (The material of the contact points affects the color). Figure 1 will illustrate the push rod and magneto shaft lever that causes the extension of the springs on the front end of the magneto. The push rod needs extend only an inch or two to stretch the springs. Upon release of the magneto shaft lever, the springs force the lever to rebound beyond the at rest position of the lever (see figure 2). The short lever at the bottom of the magneto shaft lever then snaps the outer igniter lever, rotating the igniter shaft and inner lever to separate the contact points (see figures 3 and 4). Notice that the contact points are normally closed, with a gap of approximately 1/16 inch between the magneto shaft bottom lever and adjusting screw of the outer igniter lever (figure 1).

There is also an extension spring attached to the lever on the igniter to hold the points in the closed position. This is not shown on the illustration. There is also a device with a cam and roller to provide a retard and advance position for the timing.

If you contemplate replacing the contact points, it is well to remember that just any contact points will not do. They must be igniter contact points, or a soft metal that will permit arcing of the points. Magneto and distributor contact points have a platinum coating of extremely hard metal that prevents arcing of the points while still permitting free flow of current. An igniter depends on arcing of the points to produce the spark. This also indicates the reason why the manufacturer recommended cleaning the points at least every thirty days.

Place screwdriver over magneto shaft end nut and under the inner right side tension spring pin. Push down on screwdriver, to extend spring. Release by pulling screwdriver handle towards you.

This is the first of two articles which Bud Motry has written on magnetos in response to our readers' inquiries. The second, on low tension magneto timing, will appear in a subsequent issue. Contact Motry at 20224 Arthur Road, Big Rapids, MI 49307.