NGK Surface Discharge type Spark Plug
True surface discharge or surface gap spark plugs have no side electrode, instead utilizing the entire face of the plug shell as a ground to ignite. The gap remains constant through the plugs entire life. They have no given heat range as the electrode design prevents the firing tip from overheating, and the insulator is flush with the metal shell to dissipate heat quickly. Therefore, these plugs are susceptible to fouling in cold applications.
SURFACE DISCHARGE PLUGS MAY BE REQUIRED IN HIGH COMPRESSION APPLICATIONS OR WITH HIGH ENERGY IGNITION SYSTEMS. Features