point opening for the No. 1 cylinder, move the magneto out  of position far enough to permit turning the magneto drive  shaft. Then, install the magneto in position again and repeat  the previous check for point opening.  Install the magneto attaching nuts on the studs and tighten  slightly. The nuts must not be tight enough to prevent the  movement of the magneto assembly when the magneto  mounting flange is tapped with a mallet. Reconnect the timing  light to the magneto and breaker points. With the light and  ignition switch turned on, rotate the magneto assembly first  in the direction of rotation and then in the opposite direction.  This is done to determine that the points just opened. After  completing this adjustment, tighten the mounting nuts. Move  the propeller one blade opposite the direction of rotation and  then, while observing the timing light, move the propeller  in the direction of rotation until the prescribed number of  degrees ahead of top dead center is reached. Be sure that the  lights for both sets of points come on points open, within the  prescribed timing position.  The magneto must be timed by first adjusting or checking  the internal timing with the magneto off the engine. This is  done by checking and adjusting the ignition points to open  at the E-gap position. The chamfered tooth should line up  (reference timing mark for the magneto) in the middle of the  timing window. The magneto is set to fire the No. 1 cylinder.  Remove the most accessible spark plug from the No. 1  cylinder. Pull the propeller through in the direction of rotation  until the No. 1 piston is coming up on the compression stroke.  This can be determined by holding a thumb over the spark  plug hole until the compression air is felt. Set the engine  crankshaft at the prescribed number of degrees ahead of true  top dead center as specified in the applicable manufacturer’s  instruction, usually using the timing marks on the engine.  With the engine set at a prescribed number of degrees ahead  of true top dead center on the compression stroke and with  final movement of the engine stopped in the direction of  normal rotation, the magneto can be installed on the engine.  [Figure 4-46]  While holding the magneto drive in the firing position for the  No. 1 cylinder as indicated by the alignment of the reference  marks for the magneto, install the magneto drive into the  engine drive. It should be installed in the middle of its slotted  flange to allow for fine timing of the magneto to the engine.  Attach a timing light to both magnetos. With the engine  still in the firing position, the magnetos should be timed by  moving them in the flange slots until the breaker points in the  magneto just open. If the slots in the mounting flange of the  magneto do not permit sufficient movement to effect breaker  Both right and left sets of breaker points should open at the  same instant, proper magneto-to-engine timing exists, and all  phases of magneto operation are synchronized. Some early  engines had what was referred to as staggered timing where  one magneto would fire at a different number of degrees  before top dead center on the compression stroke. In this  case, each magneto had to be timed separately. In the following example, a timing light is used for timing the  magneto to the engine. The timing light is designed in such  a way that one of two lights come on when the points open.  The timing light incorporates two lights. When connecting  the timing light to the magneto, the leads should be connected  so that the light on the right side of the box represents the  breaker points on the right magneto, and the light on the left  side represents the left magneto breaker points. The black lead  or ground lead must be attached to the engine or an effective  ground. When using the timing light to check a magneto in a  complete ignition system installed on the aircraft, the ignition  switch for the engine must be turned to both. Otherwise, the  lights do not indicate breaker point opening.  The ignition system has checks performed on it during the  aircraft engine run-up, which is the engine check before  each flight. The magneto check, as it is usually referred to,  is performed during the engine run-up check list.  One other check is accomplished prior to engine shutdown. The ignition system check is used to check the individual  magnetos, harnesses, and spark plugs. After reaching the  Performing Ignition System Checks 4-27 Figure 4-46.  Timing marks aligned. 1.  The internal timing of the magneto, including breaker  point adjustment, which must be correct to obtain  maximum potential voltage from the magneto. 2.  The engine crankshaft position where the spark occurs.  The engine is usually timed by using the No. 1 cylinder  on the compression stroke.