The ignition coil transforms the battery's 12 volts into the high voltage (approximately 20,000 volts) needed to create a spark at the plugs. The coil must be specified for the correct polarity, positive earth coils have the SW and CB terminals marked for the opposite convention to negative earth types. Fitting a coil of the wrong polarity reduces spark energy by approximately 25%, causing difficult starting and misfiring. On the Twin Cam, the ignition cables were lengthened and re-routed around the front of the cylinder head from engine number 556.
HT Leads
The HT (high tension) lead set comprises the lead from the coil to the distributor cap centre terminal and four leads from the cap to the spark plugs. HT leads deteriorate with age, the insulation breaks down, allowing the spark to track along the outside of the lead or to adjacent components rather than reaching the plug. This causes misfiring, particularly in damp conditions. Carbon-core leads provide radio interference suppression and are the standard replacement.
Silicone leads offer improved heat resistance and longer service life.
Ordering Notes
Specify positive or negative earth for the coil. HT lead sets are available for both pushrod and Twin Cam applications, the Twin Cam set has longer leads with different routing.