Magneto or Distributor

This is a stupid question but I have no mechanical skills. How do I tell if I have a magneto or a distributor?

Very simple. If it has a coil with a wire going to a 5 nipple distributor cap then it is a distributor. If it does not have that then it is a mag

This is a distributer:





This is a Magneto: (Warning: BIG PIC)

http://external_link/gallery/McCormick-Deering_Farmall_H_with_Magneto.jpg

Assuming you're talking about a letter series tractor or a Cub, so here's a smaller picture of an H4 magneto, which was the standard mag for the letter series. The J4 on the isn't much different.

Yours may be painted.

A battery ignition will have an external coil like Mike shows and old describes, with the one large wire from the coil to the center of the distributor cap. On the H/J4 mags the coil is internal, and that wire runs to the cap from the end of the other, more rectangular, cap on top of the mag.

Careful. If the internal coil on a magneto has gone bad, an external coil could have been used.

If you can identify which of the two you have from earlier posts, you can safely ignore the rest of this.

To address Trent's concern . . . and before you're completely confused . . .

Of the two types of ignitions, battery and magneto, both have distributors, It's the part of the whole mechanism that includes a shaft, points, cap and rotor and distributes the spark around to the various plug wires.

In a conventional battery ignition that was original to a letter series Farmall, the distributor is simply that. It is geared to the cam and has internal gears in its mount, all so that everything turns at the correct speed and in time. In this setup, the coil is always external. Power to the coil comes from a wire connected (sometimes through several connections, but connected nonetheless) to the battery. The condenser was originally and usually is internal to the distributor, in the same area as the points, but you'll come across the occasional modification and find it external.

The original magneto is geared in the same manner to the cam and for rotation speed. The difference is what is in between the motor and distributor. Instead of a simple mount/gear housing, there lies the actual magneto, within the shiny metallic part in the picture I posted. It consists of a magnetized shaft spinning inside a magnetized armature and a coil to pick up the field that will generate the spark. The points and condenser are internal, much the same as in the battery ignition (though slightly different), and located inside a cover under the distributor cap.

Ther are other internal differences, like the advance mechanism, but that's nothing that would help you identify what you have.

The modification Trent refers to is sometimes found in the case of a magneto that has failed because of weak magnets, bad coil . . . It was a down and dirty fix that bypasses the magneto entirely. Basically it makes use of the distributor on the face of the magneto but is wired so that the field that makes the spark is built inside an external, battery supplied coil, instead of the magnets and coil in the magneto.

So . . . to your basic question, If what originally came on your tractor is still on it, you should be able to identify it from the pictures and the basic descriptions others gave earlier and ignore all of this.

If it is different from either, come on back with a description of what looks different. If you seem to have a magneto, but see an external coil, then it likely was modified as Trent speculated. There were also battery distributors available that were oriented vertically, as opposed the the stock horizontal setup in Mike's picture. There were other makes of magneto (Wico, I believe, but maybe FM) which weren't original to the letter series but could be bolted right on and used.

Start with the basic two. They'll cover 90% of cases.