Building and Using a Magneto Timer

Understanding Magneto Timing is Key to Proper Engine Operation

| November/December 2003

Magneto Timer Schematic

Diagram #1: Magneto Timer Schematic

Every so often, we read someone's request for help in timing a magneto to their engine. This isn't a particularly difficult job, but it can be made a little easier with the right tools and a bit of understanding.

In the high-tension magneto, the spark occurs when the breaker points open, just as it does with a coil and battery system. For the hottest possible spark, the points must open exactly at the magnetic neutral, or 'E-Gap' position of the armature. E-Gap is usually found by the alignment of marks on the gears inside the magneto or by a degree measurement made with a special fixture. The onset of breaker point opening at E-Gap determines the final point gap setting. For the engine to run properly, this hot spark must occur at a certain time in the piston's travel, usually several degrees before top dead center. Both types of timing depend on knowing exactly when the points open. While this may be accomplished by the old-time technique of placing a bit of cellophane between the points and noting the position of the timing mark at the time of the release of the cellophane, there is an easier and more accurate method: the aircraft mechanic's electronic magneto timer or 'buzz box.'

Virtually all piston aircraft engines use dual magneto ignition, and the magnetos must be properly timed, both internally and to the engine. The buzz box allows the mechanic to accomplish this quickly and accurately. Aircraft buzz boxes have two little lights to allow simultaneous timing of the two magnetos, but since this is not needed for a single ignition engine, the plans show how to build a one-magneto system timer.

Necessary components
These timers are available commercially from aircraft tool supply houses, but a suitable box may be simply constructed from materials often found in the well-equipped engine buff's shop. The following materials are needed:

Transformer: Table radio (tube-type) speaker output transformer, 5-watt, 3,500 ohm/3.2 to 4 ohms. Called 'T-92 Output.' May have numbers: 7483-1, 7668-1, 7842-1, 7510, 7495, 7743-1, 7244, 7590-1, 7947 or 7712-1.

Lamp: NE-51H neon lamp or any 125th watt such as NE-2, NE-48 or NE-51 without resistor. Operates on about 60 volts AC.