Magneto Timing: Definition, Function, Tool, Procedure, Adjustment

Magneto timing is split into two parts: internal and external. Inside every magneto the points open at a fixed angle that is set on the bench; this internal setting is printed on each magneto's data plate. Once bolted to its accessory housing the magneto is rotated until the impulse coupling snaps, the mag is secured, and the fixed spark is delivered at the angle stamped on the engine data plate - usually 20-28 BTDC.

External timing is accomplished by rotating the magneto on its pad until the point-opening event occurs at the crank angle recorded on the engine data plate. A timing pin in the accessory housing or in special tools like the T-118 (Slick) or Mag Lock (Bendix) prevents movement while the nuts are tightened. The spark leaves the magneto at the instant the cam follower crosses the high spot, the impulse coupling snaps, and the distributor sends high-voltage pulses to the plugs so that combustion pressure rises just after the piston passes TDC.

Annual inspections include a mag check with a timing light which proves that spark fire occurs at the required crank position. Any drift is corrected by loosening the flange nuts and repeating the external timing procedure. During overhaul the magneto is disassembled on the bench, so every part is inspected and the internal timing (point gap and E-gap) is reset before the magneto is returned to the engine and clamped in position.

Expert behind this article

Jim Goodrich

Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.

What is magneto timing?

Magneto timing is the process of adjusting each magneto so that its primary-coil collapse occurs when the crankshaft is about 25 degrees before top dead center, the point where the greatest magnetic field stress - called E-gap - occurs and the mags fire at that point. Magneto timing involves two timing operations which need to be completed: an internal magneto timing and a magneto-to-engine timing. The main difference is that magnetos have "fixed" timing, and the act of firing the spark plugs by the magnetos is when the magneto points open. The crankshaft is set at the position that corresponds with the desired timing, and magneto timing will be printed on the engine's data plate.

What is the purpose of magneto timing?

The purpose of magneto timing is to have the breaker points open when the magnetic field induced in the coil by the rotor is at its maximum. At this instant, known as the E-gap, the greatest magnetic field stress occurs, so the coil delivers the strongest possible spark to the plugs and combustion proceeds with full efficiency.

Correct timing is therefore pivotal. Incorrect timing causes detonation and engine damage. Detonation causes pistons to melt and cylinder heads to blow, while late timing robs power and overheats the cylinders. Proper adjustment also guarantees that both magnetos fire together. During the check the technician must verify that the mags are actually

firing at that point and together so the engine runs smoothly and magneto failure is prevented.

How does magneto timing work?

The working of a magneto timing involves a high-voltage magneto converting mechanical rotation into high-voltage pulses that are used to fire the spark plugs without external power. Inside the magneto, a rotating wiper on the distributor gear turns at one-half crankshaft speed, while the high-tension lead of the coil is connected to this rotating wiper. Breaker points opening interrupts current flow in the primary coil winding, causing the magnetic field in the coil core to collapse. The coil’s primary winding stores energy then releases it as a spark. Spark plug fires when high voltage is applied.

For dual installations, a timing synchronizer is used. After the timing synchronizer is in place, you stand at the #2-#4-#6 cylinder side and rotate the engine until the impulse coupling trips at 150 RPM. Cylinder 1 is the timing reference. The propeller moves in the direction of the normal engine operation. When the magneto points open, the buzz box lights on and the mags fire. The engine backs up about 30 degrees after impulse coupling trips. On the next flight, the electronic ignition is shut off and everything is stabilized.

Magneto timing works by aligning the instant the inner armature creates its high electromotive force with the piston's position near top-dead-center. A timing flash attached to the spark coil place reveals whether the flash is coming too soon or too late. If the flash is coming too soon, the hazard of disturbing pre-ignition lifts, the motor would recoil, and the engine becomes a spattering mess. If the flash is coming too late, decelerating it cools the motor yet sacrifices economy and strength. By orienting the ignition's inner distributor while watching a line on the flywheel line up with a fixed indicator, the operator can advance or decelerate the temporal order until the engine changes to a sleek, potent point.

What is the internal timing of a magneto?

A magneto’s internal timing is the alignment of the various moving internal components of the magneto, which is set by the way the magneto itself is assembled and is not the same as ignition timing of the engine. Internal magneto timing usually only needs to be done at installation or removal and replacement, like for a leaking magneto gasket.

Magneto internal timing is the alignment of the breaker points, cam and rotating magnet so that the primary current is interrupted exactly when the magnetic flux change is greatest, producing a spark of maximum intensity at the instant the distributor finger routes the surge to the correct cylinder lead. This alignment is set entirely inside the magneto and therefore has no direct relation to crankshaft position or engine ignition timing.

The pivotal element in internal timing is the E-gap, the precise angular distance the rotor must travel past the neutral or "dead" magnetic position before the points snap open. Setting the E-gap correctly ensures that the collapsing field occurs when the induced voltage is highest, while an incorrect setting weakens the spark or alters the instant of spark delivery. Because the E-gap value is established by the points-follower profile on the cam, any wear on the follower or repositioning of the breaker assembly changes the opening instant and thus the internal timing. The timing must therefore be reset whenever the magneto is removed, installed or overhauled.

Internal timing is a two-step procedure. First the stationary point gap is adjusted to the specified thickness so the rubbing block is seated properly on the cam, then the entire breaker assembly is rotated until the E-gap, measured in degrees of rotor travel, matches the figure in the magneto data plate. Once both settings are verified, the magneto is closed, and the external timing - its angular relationship to the engine crankshaft - is checked and adjusted on the accessory case without disturbing the internal arrangement.

What is a magneto distributor block?

A magneto distributor block is the stationary section of the distributor that is housed inside the magneto and contains the breaker assembly, the rotating magnet, and the coil. Together with its gear, it forms a rotating assembly whose job is to release every voltage spike and direct the resulting high-voltage electricity to the correct spark plug at the correct instant. The block carries as many electrodes as there are engine cylinders, and these electrodes are numbered consecutively in the direction of distributor rotor travel so that the firing order is preserved. High-tension wires run from the electrodes to the respective plugs. The block and its gears are molded from qualified brown or black Thermoset Bulk Molding Compounds-non-conductive material capable of withstanding the 20,000-volt secondary voltage without creating a stray conducting path.

What is a magneto timing tool?

A magneto timing tool senses changes to the magneto magnetic circuit and uses a very low voltage. Inductive magneto timing tools are available in solid-state type or mechanical contactor type. A magneto timing tool is any device that lets a mechanic find the exact instant the magneto points open so the ignition can be set to the engine's crankshaft position. The simplest form is the buzz box, a small aluminum circuit tester with lights and a tone that connects the P-lead to ground. Its lights turn off and its tone stops the moment the points open, giving a clear visual and audible signal. Because it measures only pure inductance instead of continuity, it works in aircraft engines, motorcycles, and other magneto-fired power plants.

One step up is a timer like the Eastern Technology Corporation Model E-25 Timing Indicator. It uses the same inductance-change principle but packages it in a hand-held box that comes with instructions printed on the tool. Kits sold for C-85, C-90, and O-200 engines include this timer, leads, and a slotted plate that fits over the prop nuts so the propeller can be moved slowly in the direction of normal engine operation while the mechanic watches the indicator.

The best magneto timing tool today is the ATS E50A Magneto Synchronizer, a modern redesign of the classic test instrument aviation mechanics have relied on for more than 50 years. Offered by Aircraft Tool Supply Company, the all-solid-state E50A uses very low-voltage signals to measure the inductance of the magneto coil. It precisely detects the moment the points break, shows left or right sync with a dual-tone audio output, and warns when the internal 9V battery runs low. Unlike a simple buzz box, the E50A is not a continuity tester: it can detect internal magneto problems including a short or open circuit and will not respond if the magneto has very low inductance, making the test both faster and more accurate.

A complete test set, often called a magneto timing box, includes the Dual Magneto Synchronizer with Tweeter which adds an extra audible tweeter for noisy hangars and works with impulse-coupled, shower-of-sparks, and boost systems. Whether the mechanic chooses the basic buzz box, the intermediate E-25 timer, or the E50A synchronizer, every tool connects the engine ground to the magneto condenser terminal, is constructed from aluminum for durability, and allows the engine to be backed up about 30 degrees after the impulse coupling trips so the timing can be checked and adjusted without removing the magneto from the aircraft.

What is a magneto timing disk used for?

A magneto timing disk is used to determine the exact rotational position of the crankshaft and, through it, the angle of crankshaft rotation from piston-stop-plug contact to true top-dead-centre (TDC). Clamped to the propeller with rubber bands, the moveable disk shows degrees prior to TDC at the pointer tip, so the operator can find any specific degree location among the 360 degrees available while the propeller is rotated slowly in the direction of normal engine operation. Because one cylinder is the timing reference, the procedure starts by inserting a piston-locating plug in the top plug hole of number-one cylinder. The timing disc - and its indicator installed on the crankshaft flange - then locates the piston accurately with relation to its position in the cylinder. While the timing-disc support must be stationary throughout the timing procedure, the disk itself is turned until the desired ignition angle is reached. The buzz box is clipped on, and both lights turning off indicate the magnetos fire together and the hold-down nuts are finally tightened using an even-step torque procedure. One reason why the much cheaper timing-disk procedure is so popular is that it performs the same task as expensive special tools while using only a timing synchronizer, a protractor-and-pointer indicator, and the engine's own accessory gearing to spin the rotor.

How to time a magneto to an aircraft engine?

While timing a magneto, before any lead is touched, the propeller is rotated in the direction of normal engine operation until the impulse couplings have snapped. Timing begins by inserting the timing pin into the L hole, after which the magneto is secured to the engine but not tightened down completely. Clamps are then loosened so the mag can be rocked gently up and down until it drops into place. Once both magnetos align with the engine at that point, tighten the hold-down nuts using an even step-torque procedure, lock down the left mag, tighten clamps on both magnetos, then remove the timing pin and reinstall the ignition leads so the engine is ready for synchronizer check.

When I attempted to time the magneto, I first opened the service handbook to follow proper instructions. I slowly turned the propeller as indicated up until the temporal arrangement symbols lined up at the proper place. I collected the spanner range, fastened the bolts, and set the mounting screws. I then did a final check.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

How to set magneto timing on Lycoming engines?

To set magneto timing on Lycoming engines, find top dead center compression stroke for cylinder #1. Rotate the crankshaft until pressure pushes the thumb away, then continue slowly until the ignition timing pointer and the split line of the two crankcase halves align. Remove the pin before you attempt to set external timing. Verify that the ES118T Timing Pin has been inserted correctly. Install magneto mounting clamps loosely to hold magneto in place, then torque magneto mounting clamp nuts to 190 to 220 in. lb.

I started by turning the motor until the number one plunger attained dead midpoint on the pressure stroke. The objective was to line up the coordinate symbols on the ignition mechanical mechanism exactly with the sign on the motor container, and I confirmed the place with a coordinate marker embedded into the accessory kit. I twisted the ignition by arm, sensed for the precise location, then fastened the ignition in part. I attached a timing flash and witnessed the timing line light up in the timing illumination's ray. The discharge happened precisely at the nominal amount of degrees before the upper dead midpoint.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

How to adjust magneto timing?

To adjust magneto timing, connect the timing light to #1 or #4 spark plug wires then start the engine and watch the strobe. If the flash does not line up with the timing mark, rotate the magneto body until the marks coincide, then tighten the magneto mounting bolts. To check magneto timing with a multimeter set to ohms mode, first disconnect the high-tension lead. Meter's probes are touched to each terminal on the secondary coil. The reading on the multimeter display indicates ohms present in the magneto coil circuit. A steady resistance within manufacturer limits confirms the coil is sound and the breaker points are ready for adjustment. When you adjust the magneto timing step, set the crank at the specified advance angle, place a thumb over the spark plug hole of cylinder one to feel compression, then observe the breaker. Contact points opening at .012" gap happens at the proper time whereas .025" gap results in points opening sooner, so rotate the magneto body until the points just crack open at the correct crank position. Tighten magneto mounting bolts, recheck with timing light and, if necessary, repeat until the strobe confirms the mark is dead on.

What happens when you advance the magneto ignition timing?

Advancing the magneto ignition timing increases high-end power because the earlier spark gives the burning mixture more time to push against the piston before the exhaust port opens. Yet, advancing ignition timing reduces low-end power and cranking ease, so producers set a compromise gap. The standard contact-point setting .012" results in points opening at the proper time for starting and mid-range work; opening them farther makes the spark arrive sooner.