A sectional view of the typical ignition lead is shown in  Figure 4-12 . Ignition leads must be routed and clamped  correctly to avoid hot spots on the exhaust and vibration  points as the leads are routed from the magneto to the  individual cylinders. Ignition leads are normally of the  all-weather type and are hard connected at the magneto  distributor and affixed to the spark plug by threads. The  shielded ignition lead spark plug terminal is available in all- weather ¾ inch diameter and  5 ⁄ 8  inch diameter barrel ignition  lead nut.  [Figure 4-13]  The  5 ⁄ 8  – 24 plug takes a ¾ wrench  on the lead nut and the ¾ – 20 plug takes a  7 ⁄ 8  wrench on the  lead nut. The ¾ inch all-weather design utilizes a terminal  seal that results in greater terminal well insulation. This  is recommended because the lead end of the spark plug is  completely sealed from moisture.  An older radial engine type of ignition harness is a manifold  formed to fit around the crankcase of the engine with flexible  extensions terminating at each spark plug. A typical high- tension ignition harness is shown in  Figure 4-14 . Many older  single-row radial engine aircraft ignition systems employ a  dual-magneto system, in which the right magneto supplies  the electric spark for the front plugs in each cylinder, and the  left magneto fires the rear plugs.  All units in an aircraft ignition system are controlled by  an ignition switch. The type of switch used varies with the  number of engines on the aircraft and the type of magnetos  used. All switches, however, turn the system off and on in  much the same manner. The ignition switch is different in at  least one respect from all other types of switches: when the  ignition switch is in the off position, a circuit is completed  through the switch to ground. In other electrical switches, the  off position normally breaks or opens the circuit. The ignition switch has one terminal connected to the primary  electrical circuit between the coil and the breaker contact  points. The other terminal of the switch is connected to the  aircraft ground structure. As shown in  Figure 4-15 , two ways  to complete the primary circuit are: Ignition Switches 4-9 Figure 4-12.  Ignition lead. Figure 4-14.  Accessory-mounted nine cylinder engine ignition  harness. Figure 4-13.  Ignition lead spark plug end.  Right magneto Switch Booster Left magneto Insulator Blue coating Conductor Sheiding