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.