# 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

LetterThread ØHex
A18 mm25.4 mmB14 mm20.8 mm
C10 mm16.0 mmD12 mm18.0 mm
E8 mm13.0 mmJ12 mm18.0 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)

LetterThread reachE19.0 mmH12.7 mmL11.2 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.

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