How to read a Champion spark plug reference
Each letter and number in a Champion code is coded: resistor, thread diameter, reach, heat range, firing end and gap. Here is how to decode it.
A Champion reference such as RN9YC is read position by position, following Champion's official nomenclature.
1. Leading R — resistor
A leading R means a built-in resistor (suppressed), e.g. RN9YC. No thread letter is "R", so a leading R always marks the resistor.
2. Thread letter(s) — diameter & reach
| Letter | Thread Ø | Reach |
|---|---|---|
| A | 12 mm | 12.7 mm |
| C | 14 mm | 12.7 mm |
| D | 18 mm | 11.7 mm |
| F | 18 mm | 12.7 mm |
| J | 14 mm | 9.5 mm (taper seat) |
| L | 14 mm | 12.5 mm (taper seat) |
| N | 14 mm | 19 mm (projected) |
| P | 12 mm | 9.5 mm (taper seat) |
Two-letter forms exist too (AX, CJ, DJ, DZ, EJ, MJ). The seat is flat (with gasket) unless the letter denotes a taper seat (e.g. J, L, P).
3. The number — heat range
The digit(s) give the heat range: lower = hotter, higher = colder. It is relative to Champion.
4. Firing end / design (final letters)
- Y — projected tip
- C — copper core
- P — platinum
- G — gold-palladium
- LM — lawn-mower type
5. Tightening torque
Torque depends on the seat, thread diameter and cylinder-head material. Flat seat (with gasket): 12 mm 15–25 N·m (iron) / 15–20 (alu), 14 mm 25–35 / 25–30, 18 mm 35–45 / 35–40. Taper seat: 12–14 mm 10–20 N·m, 18 mm 20–30. Always follow the value printed on the box.