Magnetos: Installation Tips

Magnetos have been around since the early 1900s and have been refined for the last 100 years to give us the reliability we have come to expect from aircraft magnetos. As maintenance professionals, it is important that we understand what makes a magneto...

First, remove the white plastic distributor cover and save the screws. Install the T-116 timing pin into the hole on the distributor block marked “L” or “R” depending on the rotation of the mag. Look at the magneto data plate to determine the direction of rotation as pictured in the photo on the opposite page.

It does not matter which direction your engine turns — look at the magneto data plate to determine where to put the pin.

That Darn Pin

The pin has to be inserted in the correct hole and inserted completely or the mag just will not work. In order to accomplish this, turn the magneto input shaft to align the pin hole in the distributor block and distributor gear. Impulse coupling magnetos should be turned backwards to prevent any tension from building up on the impulse coupling spring when the pin is installed.

There is an electrode between the distributor block and the distributor gear that may contact the timing pin when you are trying to install it. Do not be fooled by this electrode. You may need to lift the pin out about ¼ inch while you continue to turn the magneto shaft where it clears that electrode just enough to put the pin down and get it fully seated. Make sure the pin is fully seated. The pin will go all the way down to the first shoulder when correctly installed.

Preparing the Engine

I don’t know how many times I’ve heard of magnetos being installed on top dead center of the exhaust stroke, and the mechanics swear on a stack of bibles that it is on the compression stroke. Don’t tell anyone, but I’ve done it myself. Here’s how to verify that the engine is ready.

First, remove the top spark plugs from all of the cylinders. Then, turn the propeller in the normal direction of rotation with your thumb over the spark plug hole on the No. 1 cylinder. When the air pressure on the No. 1 cylinder starts to build up and tries to blow your thumb off the hole, slowly continue to turn the prop until the timing mark listed on the engine data plate lines up exactly with the split line on the top of the crankcase for Lycoming engines or the split line on the bottom of the crankcase for Continental engines. Typically, Lycoming engine timing marks are located on the starter ring gear and Continental engine timing marks are located on the propeller flange. A degree wheel is highly recommended on Continental engines. I spent six months in the assembly room at Teledyne Continental Motors. During that time these professional assemblers never once installed a magneto without using a degree wheel. It would be pretty arrogant for a mechanic that installs a magneto once or twice a month to assume that he or she could accurately install a magneto on a Continental engine without using a degree wheel.

The engine will be on the compression stroke of the No. 1 cylinder, and you are now ready to install the magneto.

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