# How to read an Eyquem spark plug reference

> An Eyquem code encodes the resistor, the shell/thread, the electrode material, the heat range, the reach, the seat, the insulator nose and the ground electrodes. Here is how to decode it.

An Eyquem code such as **RFC 42 LS** is read position by position: **R** (resistor) · shell/thread letter · electrode material · **heat range** · reach / seat / nose / ground-electrode letters.

## 1. R — resistor

**R** at the front = built-in resistor (3–9 kΩ); no letter = no resistor.

## 2. Shell type — thread

- no letter / D / Z / W / F — **M14×1.25** (flat or taper seat)
- **K** — M18×1.5
- **T** — M10×1.0
- **X** — M12×1.25 (Bi-Hex)

## 3. Electrode material

- no letter — nickel centre & ground
- **C** — copper-cored centre electrode
- **N** — copper-cored centre & ground electrodes

## 4. Heat range

Super range: **12 = hot … 99 = cold**. Nickel/Chrome range: 200 (hot) … 1200 (cold). Relative to Eyquem.

## 5. Reach, seat, nose & ground electrodes

- Reach: no letter 11.2 mm · **A** 7.8 · **C** 17.5 · **L** 25 mm
- Seat: no letter flat · **J** taper seat
- Insulator nose: no letter not projected · **S** projected · **Z** projected + anti-fouling
- Ground electrodes: no letter/1 = one (standard) · **2 / 3 / 4** = number of side electrodes

## 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 an Eyquem spark plug reference](https://www.thesparktwin.com/en/guide/nomenclature-eyquem/) — TheSparkTwin*