How to read a Bosch spark plug reference
Each letter and number in a Bosch code is coded: thread & seat, type, resistor, heat range, reach, electrode and gap. Here is how to decode it.
A Bosch reference such as FR7DC+ is read position by position, following Bosch's official type formula.
1. First letter — thread, seat & spanner
| Letter | Thread | Ø / seat |
|---|---|---|
| A | M12×1.25 | 12 mm · taper |
| D | M18×1.5 | 18 mm |
| F | M14×1.25 | 14 mm · flat |
| H | M14×1.25 | 14 mm · half-thread |
| M | M18×1.5 | 18 mm · flat |
| N | M10×1.0 | 10 mm (small engine) |
| W | M14×1.25 | 14 mm · flat |
| U | M10×1.0 | 10 mm |
| Q | M16×1.5 | 16 mm |
| V / X / Y / Z | M12×1.25 | 12 mm |
2. Type letter
- A — EVO spark plug
- F — surface-discharge (sliding spark)
- L — semi-surface air-gap
- M — air-gap
- U — motorsport
3. R — resistor
An R in the leading letters (e.g. FR7DC) means a built-in resistor (suppressed).
4. The number — heat range
The digit is the heat range: lower = hotter, higher = colder. Relative to Bosch.
5. Remaining letters
After the heat number come the thread length / spark position, the electrode concept, the precious-metal material (I = iridium, P = platinum), the version, terminal and gap. A trailing + marks a Bosch «Super Plus».
6. 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. Always follow the value printed on the box.