Accurate magneto-to-engine timing is essential for proper ignition system operation and engine performance. Several timing references and tools are used to establish the correct crankshaft position and determine the exact point at which the magneto breaker points open. These include built-in engine timing marks, timing discs, piston position indicators, and timing lights.
Built-In Engine Timing Reference Marks
Most reciprocating engines have timing reference marks built into the engine. The timing reference marks vary by manufacturer. [Figure 1]
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| Figure 1. Lycoming timing marks |
When the starter gear hub is correctly installed, the timing marks on the hub align with a reference mark on the starter. On an engine that has no starter gear hub, the timing mark is normally on the propeller flange edge. [Figure 2]
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| Figure 2. Propeller flange timing marks |
The top center (TC) mark stamped on the flange edge aligns with the crankcase split line below the crankshaft when the No. 1 piston is at top dead center. Other flange marks indicate degrees before top center.
Some engines have degree markings on the propeller reduction drive gear. To time these engines, the plug provided on the exterior of the reduction gear housing must be removed to view the timing marks. On other engines, the timing marks are on a crankshaft flange and can be viewed by removing a plug from the crankcase. In every case, the engine manufacturer’s instructions specify the location of the built-in timing reference marks.
When using built-in timing marks to position the crankshaft, sight directly across the stationary pointer or reference mark and the corresponding timing mark. [Figure 3]
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| Figure 3. Typical built-in timing mark on propeller reduction gear |
Sighting at an angle results in an error in positioning the crankshaft. Normally, the No. 1 cylinder is used when installing or checking magneto-to-engine timing. When installing magnetos, the applicable timing marks must be correctly aligned, and the No. 1 cylinder must be on its compression stroke.
The amount of gear backlash varies between installations because clearance exists between the gear teeth. To minimize timing errors caused by backlash, always approach the specified timing position by rotating the engine in its normal direction of rotation. Another potential source of error when using timing marks on a reduction gear is parallax. This can occur when sighting between the external reference mark and the timing mark inside the reduction gear housing because the two marks are located at different depths.
Timing Discs
Most timing disc devices are mounted on the crankshaft flange and use a graduated timing plate. [Figure 4]
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| Figure 4. A timing plate and pointer |
The markings vary according to the specifications of the engine. The plate is temporarily installed on the crankshaft flange and contains a scale graduated in crankshaft degrees, with a pointer used to indicate crankshaft position.
Piston Position Indicators
Piston positions used for ignition, valve, or injection pump timing are referenced to top dead center (TDC). For precise timing purposes, top dead center should not be confused with the piston position referred to as top center. Near the top of its travel, the piston may show little or no noticeable movement while the crankshaft rotates through approximately 1° to 5°. Therefore, the general top-center position has limited value as a precise timing reference. This condition is illustrated in Figure 5, which is exaggerated to emphasize the piston’s no-travel zone.
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| Figure 5. Difference between top center and top dead center |
Notice that the piston shows little or no movement while the crankshaft rotates through the small arc from position A to position B. This no-travel zone begins when the crankshaft and connecting rod stop moving the piston upward and continues until the crankshaft has rotated far enough for the connecting rod to begin moving the piston downward.
Top dead center (TDC) is the precise piston-and-crankshaft position from which other piston and crankshaft positions are referenced. At TDC, the piston is at the center of the no-travel zone, and a straight line passes through the centers of the crankshaft journal, crankpin, and piston pin. This alignment is shown on the right side of Figure 5. In this position, a force applied directly to the piston cannot produce a turning moment on the crankshaft.
Timing Lights
Timing lights are used to determine the exact instant at which the magneto breaker points open. There are two general types of timing lights in common use. Both have two lights and three external wire connections. Although both have internal circuits that are somewhat different, their function is essentially the same. [Figure 6 and 7]
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| Figure 6. E50 Magneto Synchronizer |
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| Figure 7. Timing light |
Three wires plug into the light box. [Figure 7] There are two lights on the front face of the unit, one green and one red, and a switch to turn the unit on and off. To use the timing light, the center lead, which is black and marked “ground lead,” is connected to the case of the magneto being tested. The other leads are connected to the primary terminals of the breaker point assemblies in the magnetos being timed. The color of the lead corresponds to the color of the light on the timing light.
With the leads connected in this manner, the technician can determine whether the breaker points are open or closed by turning on the timing light and observing the two indicator lights. On the type of timing light described here, the lights remain off when the breaker points are closed because most of the current flows through the points rather than through the timing light circuit. When the breaker points open, current flows through the timing light circuit and the corresponding indicator light illuminates. Some models operate in the reverse manner; on these units, the indicator light goes out when the breaker points open.
Each indicator light operates independently and corresponds to the set of breaker points to which it is connected. This allows the technician to determine the exact point in magneto rotor rotation at which each set of breaker points opens.
Most timing lights use batteries that require periodic replacement. Using a timing light with weak batteries may produce erroneous readings because of insufficient current flow within the unit.