How to read an NGK spark plug reference
Every letter and number in an NGK code is coded: thread diameter, reach, heat range, electrode and gap. Here is how to decode it.
An NGK reference such as BPR6ES-11 is read position by position. Each field is defined by NGK's official nomenclature.
1. First letter — thread diameter
| Letter | Thread Ø | Hex |
|---|---|---|
| A | 18 mm | 25.4 mm |
| B | 14 mm | 20.8 mm |
| C | 10 mm | 16.0 mm |
| D | 12 mm | 18.0 mm |
| E | 8 mm | 13.0 mm |
| J | 12 mm | 18.0 mm |
A letter I, P, S, Z or V placed before the thread letter is a construction prefix (see below), not the diameter.
2. Construction letters
- R — built-in resistor (suppressed)
- P — projected insulator / platinum tip
- Z — corrugated (surprojected) insulator
- U — surface / semi-surface discharge
- L — long thread (26.5 mm) or half-step hotter
3. The number — heat range
The digit is the heat range: low = hot (2, 4, 5…), high = cold (9, 10, 11…). It is relative to NGK and not comparable letter-for-letter with other brands.
4. Reach letter (after the number)
| Letter | Thread reach |
|---|---|
| E | 19.0 mm |
| H | 12.7 mm |
| L | 11.2 mm |
5. Firing-end / electrode
- I / IX — iridium centre electrode
- P / VX — platinum centre electrode
- G — fine nickel-alloy electrode
- F / FS — taper seat (no gasket)
- S — copper core
6. Gap suffix
A trailing -9 / -10 / -11 / -13 is the factory gap in tenths of a millimetre: -11 = 1.1 mm.
Tightening torque
Torque depends on the seat, thread diameter and cylinder-head material. Flat seat (with gasket): 10 mm 10–15 N·m (iron) / 10–12 (alu), 12 mm 15–25 / 15–20, 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. Or by angle on a new plug: 14/18 mm ½–⅔ turn, 10/12 mm ½ turn, 8 mm ⅓ turn. Always follow the value printed on the box.