magneto timing: internal & external

Some time ago I was recruited to be part of an NTSB investigation team looking into the cause of a loss of power on takeoff with a Jacobs 275 hp powered aircraft.  Although the aircraft was heavily damaged in landing, the passengers escaped with only minor cuts and bruises.  Our job was to try to determine what had gone wrong.  After verifying the quality of the fuel delivery system and making sure that the engine had not experienced internal failure, we turned our attention to the ignition system.

The Jacobs R755A and B Operators Manual states that the point setting for the magneto is to be .012” and .016” for the battery distributor.  Both units are to be timed to the engine at 31 degrees before top dead center (BTDC).  We found that the left side contacts were adjusted to .036” and the right to .027”, and that the left side was timed to the engine at 27 degrees BTDC with the right at 36 degrees BTDC.  Obviously these settings would not allow the engine to deliver optimum performance, but the question became “what effect did these settings have on the operation of the engine?”

Internal Timing—Adjustment of the Contact Points

In its simplest form the aircraft magneto can be considered a high voltage electrical generator that can function independently of an outside source of excitation.  As the engine turns the rotating magnet in the magneto, an alternating current is induced in the primary windings of the magneto coil.  As long as the contact points are closed, electrical energy is stored in these windings.  When the points open, this energy is released through the secondary windings of the coil as high voltage, which finds its way through the rotating cylinder to the magneto blocks, and ultimately to the spark plugs.

Figure 1

Figure 1. shows the sine wave produced by one-half revolution of the magneto’s rotating magnet.  The peaks of the sine wave, both positive and negative, are at roughly 20,000 volts and represent the peak voltage produced in the secondary windings of the coil.  The purpose of the contact points is to open when the induced voltage is the highest (at the crest of the wave) so that the hottest spark is delivered to the spark plugs.  The magneto manufacturer designed the magneto cam and contact assembly in such a way that a .012” contact gap between the contact points (when the cam follower is located on a lobe of the cam) will result in the points opening at the proper time to take advantage of the full voltage (peaks of the sine wave) being produced in the coil.

Figures 2 & 3 show the results of readjusting the contact points to first .025”, and then .035”.  As you can see, both these settings cause the points to open sooner than the .012” setting does, with a resultant loss of voltage.  As we increase the point settings we “cut off” the top of the sine wave and cause the current to flow through the secondary windings and out to the spark plugs before the rotating magnet has induced the maximum voltage.  The result is a lower voltage and weaker spark at the plugs.

Figure 2

Figure 3

External Timing—Adjustment of the Magneto to the Engine

The other adjustment that is normally checked at 100 hour and annual inspections is the timing of the magneto to the engine.  As noted above, the manufacturer gives us a setting of 31 degrees BTDC for the Jacobs 275 and 300 hp engines.  This adjustment is made by loosening the two magneto hold down nuts after the engine has been set to 31 degrees BTDC and then rotating the magneto until the contact points just open.  These are those contact points again!  Since the magneto external timing is dependent upon the contact point setting, it is necessary to always first check (and adjust if necessary) the contact points before timing the magneto to the engine.  Since the engine that was being inspected in our investigation had contact points that had two and three times as much contact gap as called out in the manual, another question that came up was, “How much change to external timing occurs due to misadjusted contact points?”  The answer was ‘not as much as I first thought it would be’.

To check this, I first set up an engine to 31 degrees BTDC, verified both sets of contact points to be sure that they were properly adjusted, and checked to see that both the magneto and distributor were in external time with the engine.  I then misadjusted the magneto points to first .025”, and then to .035”, and measured the external magneto timing at both these contact settings.  As expected, the external timing was advanced in both cases, but only to 34 degrees BTDC at the .025” contact setting, and 36 degrees BTDC at the .035” contact setting.  Though these changes were less than I expected that they might be, either setting could be very detrimental to the engine, as we will see.

The theory behind timing the combustion event is that the engine designer wanted to introduce a spark at a certain time (in our case, 31 degrees BTDC) as the piston is coming up to TDC on the compression stroke, so that the maximum pressure from combustion is created just as the piston passes TDC.  If we fire the spark plugs too late, the maximum pressure occurs some time after the piston has passed TDC and there is a power loss in the engine.  If we fire the spark plugs too soon (say 34 or 36 degrees BTDC for example) the maximum pressure occurs while the piston is still trying to reach TDC and the engine is working against itself.  At best there is a loss of power, at worst we have detonation within the engine.  These radial engines that we fly and love are very well designed and durable engines, but no engine will stand up to long periods of detonation.  At some point, pistons, link rods, and cylinder heads begin to fail.


In the engine we were looking at during our investigation, we had contact points on both the left and right sides that were opening too soon and delivering low voltage to the spark plugs.  In addition, the left side was firing late with the resultant loss of power and the right side was firing early creating not only a loss of power, but also the possibility of detonation.

When you consider the number of times per hour that the magnetos and distributors deliver a spark to begin the combustion event, you realize that they are amazingly reliable units.  However, the engines that they are installed on have been designed to operate within some fairly restricted timing settings.  Good service can be expected from these units only if both the internal and external timing are inspected regularly and maintained.