When new, your ’54 CD-11 came with a J6-J while the ’62 FD-16 probably had a colder J4-J installed in it. The hotter running thermostat temperature controlled FD has much higher compression requiring the colder plug but as the motor loses compression over time, it may require a slightly hotter J6-J plug to help keep deposits from forming and maintaining performance. With todays clean burning TCW III oils, deposits are no longer the problem they once were so the J plugs with the shortened electrode are no longer required. The correct heat range can be determined by studying a new plug after using the motor for a short time for its intended purpose. Heavy loads such as WOT or water skiing will generally require a colder plug but trolling or slower speeds might require a slightly hotter plug. The color of the plugs tip will indicate which one to use.

The Champion J4-J’s cross over to the superior NGK B6S plugs while the J6-J’s can be replaced with B8S plugs. Happy motoring!

http://www.sparkplug-crossreference.com … AMP_PN/J6J

http://www.sparkplug-crossreference.com … AMP_PN/J4J