FAA Powerplant Written Practice Test

What is the required position of the piston in the no. 1 cylinder when installing a magneto?

1. At the top dead center on the exhaust stroke

2. A few degrees before top dead center on the compression stroke

3. At the bottom dead center

4. At a random position

The correct answer is: A few degrees before top dead center on the compression stroke

The required position of the piston in the no. 1 cylinder when installing a magneto is a few degrees before top dead center on the compression stroke. This specific position is crucial because it ensures that the ignition event occurs at the proper time in relation to the piston’s movement. When the piston is near the top of its travel just before it reaches top dead center, the fuel-air mixture is fully compressed, making it ready for ignition. This timing is essential for optimal engine performance as it helps in achieving complete combustion when the spark plug fires. Installing the magneto with the piston in this position allows for the spark to ignite the fuel-air mixture just as the piston reaches the top and begins its power stroke. If the piston were at top dead center on the exhaust stroke, the engine would not be able to ignite the mixture effectively, leading to poor engine performance or even failure to start. Similarly, at bottom dead center, although magneto installation might be physically possible, the timing would not correspond to the optimal moment for ignition. A random position would not ensure proper timing or performance either, as it does not relate directly to either the compression or combustion process.