Everything You Need to Know About Magnetos PDF Print E-mail
Written by Jacqueline Shipe   
Thursday, 24 February 2011 10:00

I recently met a buddy at the airport to go with him for a flight in his Piper Cherokee Six. This was to be a test flight to make sure his engine temperatures were in the correct range. We ran the engine up to do a mag check before we took off. As he turned the key, the first position produced no change in rpm. The second position resulted in the engine cutting off completely. The right magneto was not firing at all, so we taxied back in.


Understanding Magnetos

A magneto is essentially a self contained generator which is gear driven by the engine. It mounts on the accessory case and consists of a permanent magnet which rotates, a coil, capacitor, set of breaker points, cam, distributor, and the appropriate housing and gears.

Anytime there is relative motion between a magnet and a conductor, electric voltage is produced. As the gears in the accessory case rotate the magnet in the magneto, it induces a voltage in the coil. The magnet usually has four poles. The magnet rotates inside of iron laminations that are made into the magneto housing. The center of the coil is made of iron and is installed so that it is in contact with the iron laminations that are cast into the sides of the housing. This forms a magnetic pathway. As the magnet with the four poles (alternating north and south) rotates, it produces a magnetic field through the laminations and coil center in one direction, then the other.

The center of the coil consists of the soft iron core which has thick conductive wire wrapped around it. This thick wire is called the primary winding. On the outside of the primary winding there is what’s called a secondary winding that consists of several thousand wraps of fine wire. This is molded in a plastic case with the center iron core protruding out the sides. One end of the primary winding is connected to the iron core in the center of the coil and the other end has a terminal which is connected to a set of breaker points and a condenser. One end of the secondary winding is grounded inside the coil and the other end has a contact terminal on it. This contact is for the distributor finger.

As the magnet rotates it produces a voltage in the primary winding that normally flows to ground. When the points open and the primary circuit is suddenly broken, a high voltage is produced in the secondary windings which is directed to the appropriate spark plug through the distributor. The capacitor (or condenser) is wired into the primary circuit parallel with the breaker points. This helps to prevent arcing as the points are opened. If the condenser is faulty, the points will become burned or pitted on the contacting surfaces.

Internal gearing in the magneto rotates a cam which opens the breaker points. A spring that is incorporated in the breaker point assembly keeps them closed the rest of the time. This gear mechanism also rotates the distributor finger so that it sends the current through the correct spark plug wire in the proper firing order.


Installing a New Magneto

Installing a magneto on an engine is generally not too difficult. The engine propeller is rotated until the compression stroke begins on the number one cylinder. This can be detected by removing one of the plugs and feeling for air being pushed out as the propeller is turned in the direction of rotation. Once this has been determined, the propeller is further rotated until the appropriate timing marks on the engine are aligned. The number of degrees before top dead center that the ignition should fire is stamped on each engine’s data plate (usually 20 to 25 degrees). Once the engine gears are in the proper alignment, the magneto is rotated by hand until it is set to fire on the number one cylinder position.

This is accomplished by inserting a wire into the number one hole on the distributor and holding it with pliers about a sixteenth of an inch from the case of the magneto. As the magneto is rotated in the same direction as it would rotate on the engine, a spark will occur at the gap between the wire and case as it fires at the number one position. Next, the cover plate is removed from over the timing window. The magnet is then backed up until a painted chamfered tooth on the distributor gear is visible in the window. This is the position where the breaker points are beginning to open, causing the magneto to fire. With the magneto kept in this position, it is then installed on the engine.

Slick mags come with a special timing pin which is inserted into a hole in the distributor housing. The pin won’t slide in all the way until the mag is rotated and set to fire on the number one cylinder. The pin also keeps the gears from moving as the magneto is bumped a little during installation.

Once installed, a timing light is connected to the magneto and the magneto housing can be rotated left or right to provide the final adjustments so that the points on the magneto open at the exact time the piston in the number one cylinder is in the correct position. The timing light has three leads, one for the left mag, right mag and ground. The leads connect to the mags at the P lead terminal. The P lead is the wire that connects the aircraft ignition switch to the magneto. If the P leads are connected, the switch has to be on to un-ground the mags. Once the mags are installed and timed, care should be taken to be sure the switch is turned back off and that the P leads are connected.


My buddy’s magneto had a bad coil in it, along with pitted points and eroded contacts in the distributor. After installing a new mag, we finally got to test fly his airplane for the original reason, and it turns out that his temperatures were fine.

From the July 2010 issue of Cessna Owner