HD plugs - Higher # = Colder Heat Range
American made plugs - Higher # = Hotter Heat Range
Foreign made plugs - Higher # = Colder Heat Range
18mm plugs
| Original plug | Equivalent modern replacement | Application |
| HD #3 | Champion D16, NGK A6 | Normal riding |
| HD #4 | Champion D14, NGK A7 | Fast riding |
| HD #5 | Champion D9 | Real hard riding |
14mm plugs
| Original plug | Equivalent modern replacement | Application |
| HD #3 | Champion H-10C, NGK B6L, Accel 2402A or Y2402P | Average or normal riding |
| HD #4 | Champion H8C, NGK B6L, Accel 2402 | Faster than normal riding |
| HD #5 | Champion J-6C | Hard riding |
A sparkplug cross reference chart should be simple, however I have researched several different sources and found discrepancies in cross reference to HD#'s and replacement plugs. The How To Restore Your Harley Davidson book has the equivalent modern replacements listed in reverse of the correct order I found from other sources. I want to get this chart correct, if you see something on here which is not right, please let me know and possibly provide a reference..
Press Back button or Home