When sending the high-energy burst of electricity to the spark plugs, the magneto has to do so at the right split second—every time. That requires that the mag be timed internally and externally. Setting the internal timing requires that the mag be removed from the engine and opened up. It should be performed every 500 hours of operation. External timing is done with the mags on the engine and should be done every 100 hours or at the annual inspection.
Internal timing involves setting the point gap and “E-gap” (“E” is short for efficiency). The point gap is set first—and involves rotating the drive shaft to the position where the breaker points are opened as far as they will go. The gap is measured with a feeler gauge and adjusted to the distance called for by the manufacturer.
The E-gap is the number of degrees of rotation between the magnetic neutral position of the rotor and when the points begin to open. It is prescribed by the manufacturer and the mag is set to that value.
The purpose is to have the breaker points open when the magnetic field induced in the coil by the rotor is at its maximum—to create the hottest spark.
External timing of the magnetos should be done every 100 hours or at the annual inspection, whichever comes first. The process involves pulling a spark plug in the number-one cylinder and rotating the crankshaft until the number-one piston is at the firing position specified for that type engine—typically 20 to 24 degrees before TDC. The technician then uses an ignition timing light connected to the mag’s P-lead terminal while adjusting each mag by loosening its base clamps and rotating the mag on the mounting pad until the timing light indicates that the points are just beginning to open. The technician then tightens the clamps and checks the timing again—the mag may have moved.
External mag timing should be within one to two degrees of the published spec for the engine. Historically, an analog dial indicator has been used when setting mag timing—and they are only accurate to a degree or so. More recently technicians have been using digital inclinometers, which are accurate to a tenth of a degree.
We have seen serious engine damage caused by mistiming magnetos. Advancing the timing—even a few degrees more before TDC than called for by the manufacturer—will cause the engine to run hotter, reducing the detonation margin. Timing advanced by even four or five degrees on a big-bore engine combined with a hot day has caused detonation on takeoff and engine destruction. Advanced timing can be an expensive event on even smaller engines—we’re aware of a Cessna 150 that required replacement of all four cylinders due to high CHTs because the timing was mis-set at the annual. The airplane did not have an engine monitor to warn that CHTs were unacceptably high.
Our research indicated that most mistiming occurs during an annual inspection. We recommend keeping track of EGTs and CHTs and checking that the after-annual readings are consistent with those prior to the annual. If the CHTs are higher and EGTs lower, the timing is advanced. Get the airplane straight back into the shop.
It’s common to have to adjust external mag timing slightly at the annual as timing drifts due to component wear. We recommend that your technician keep a record of the direction and number of degrees he or she “bumps” each mag at each annual. If the cumulative bump exceeds three degrees in one direction before the next 500-hour IRAN, it’s an indication that there’s a problem with the internal timing and the mag should be pulled, opened up and inspected.