# 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

LetterThreadØ / seat
AM12×1.2512 mm · taperDM18×1.518 mm
FM14×1.2514 mm · flatHM14×1.2514 mm · half-thread
MM18×1.518 mm · flatNM10×1.010 mm (small engine)
WM14×1.2514 mm · flatUM10×1.010 mm
QM16×1.516 mmV / X / Y / ZM12×1.2512 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. F**R**7DC) 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.

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*Source : [How to read a Bosch spark plug reference](https://www.thesparktwin.com/en/guide/nomenclature-bosch/) — TheSparkTwin*