The original J10 commercial J was changed (renamed) J6-J around 1946. These became J6-C when they went to a solid copper center electrode. The short plugs are the same number with a preceding C, ie. CJ6-C. From the 1949 Champion catalog – Note the C7 is a hotter plug than the 7 – later books are fuzzy on this distinction. Factory recommendations are not etched in stone. running conditions vary. check the look of the insulator. Adjust to the heat range that matches your running condition.
Garry in Michigan
