How to Read Spark Plug Part Numbers
A spark plug part number isn't a random code — for most brands it's a compressed spec sheet. Once you can read it, you can tell a plug's thread size, reach, heat range, electrode design, and factory gap without opening a catalog. Here's how each major system works, with one worked example each. (Or skip the reading and paste any number into our free decoder.)
The heat range trap — read this first
The single most dangerous mistake in plug numbering: assuming heat range digits mean the same thing across brands. They don't, and the scales even run in opposite directions:
| Brand | Scale | Higher number means |
|---|---|---|
| NGK | 2 – 12 | Colder |
| Denso | 9 – 37 | Colder |
| Champion | 1 – 25 (automotive band) | Hotter |
| Bosch | 06 – 13 | Hotter |
| Autolite | last digit within family | Hotter |
| ACDelco | 0 – 9 | Hotter (Z-types reversed) |
NGK
Classic NGK numbers read shell → construction → heat range → reach → firing end → gap. Take BPR6ES-11: B is a 14mm thread with 13/16" hex, P means projected insulator, R means built-in resistor, 6 is the heat range, E is 19mm reach, S is the standard copper-core center electrode, and -11 is a 1.1mm factory gap. Modern laser iridium plugs (like ILKAR7B11) use extended series prefixes beyond the classic chart, but the heat digit and trailing gap number still follow the same logic.
Denso
Denso follows a similar shape with its own codes: W16EPR-U11 breaks down as W (14mm × 20.6mm hex shell), 16 (heat range), E (19mm reach), P (projection), R (resistor), -U (U-groove ground electrode), 11 (1.1mm gap). Remember the scale difference: a Denso 16 is roughly an NGK 5.
Champion
Champion reads prefix → shell → heat/application → firing end → gap. RC12YC: R resistor, C 14mm × 5/8" shell with 3/4" reach, 12 heat range, Y projected core nose, C Copper Plus electrode. A trailing 4, 5, 6, or 8 indicates a factory wide gap.
Bosch
FR7DC+: F 14mm × 1.25 thread with 16mm hex, R suppression resistor, 7 heat range, D 19mm reach, C copper electrode, + Super Plus technology. Beware: Bosch also sells plugs under plain 4-digit stock numbers that aren't coded at all.
Autolite
Autolite is numeric. In 2545, the leading digits identify the plug family (plugs sharing thread, reach, and hex), the third digit's parity indicates resistor (even) versus non-resistor (odd), and the final digit is the heat range within the family. Prefixes mark the tier: AP platinum, APP double platinum, XP iridium.
ACDelco
Classic AC numbers like R46TS: R resistor, 4 14mm thread, 6 heat range, T taper seat, S extended tip.
Frequently asked questions
Are heat range numbers comparable between brands?
No — every brand uses its own scale, and half of them run in opposite directions. NGK and Denso count up toward colder; Champion, Bosch, Autolite, and ACDelco count up toward hotter. An NGK 6 is roughly a Denso 20 and near a Champion 12–14 range equivalent. Always convert through a cross-reference, never by matching digits.
Why doesn't my Motorcraft part number decode?
Because Motorcraft numbers (like SP-542) are assigned catalog numbers, not coded systems — the digits carry no specs. The same is true of many 4-digit Champion and Bosch stock numbers. For those, a cross-reference lookup is the only way to learn the specs.
What does the number after the dash mean on NGK plugs?
It's the factory pre-set gap in tenths of a millimeter: -11 means 1.1mm (.044\"), -9 means 0.9mm (.035\"). The same plug can be sold with different dash numbers for different applications.
Do modern laser iridium plugs follow the classic charts?
Only partially. Long codes like ILKAR7B11 or DILZKR7B11GS use extended series prefixes that post-date the published classic charts — the heat digit and trailing gap still follow the pattern, but some letters are series-specific. Our decoder labels those segments honestly rather than guessing.
More guides
Metal Tier Guide · Cross Reference Guide · Back to the finder & tools