I'm running stock heat range plugs in my 1971 2000cc Capri. I'm concerned that they may be running a bit hot, especially since I found some little round metallic balls on the number 4 insulator.

Anybody know how the manufacturers numbers work. For example, the motor takes a Motorcraft 42 or Autolite 45 plug. Can I specify a different number to get a colder plug that still fits?