Automotive Reference
Two things trip people up on plugs: which socket pulls them, and the gap. Here's the plug decoded, the two common socket sizes, and how to set the gap right.
You need the right socket to remove it (measured across the hex) and the right gap at the electrode for it to spark properly.
| Socket | Metric | Common on |
|---|---|---|
| 5/8" | 16 mm | Most modern plugs (the one to own) |
| 13/16" | 20.6 mm | Older / larger plugs |
| 14 mm | thin-wall | Many import 4-cyl (deep, narrow wells) |
| 18 mm | thin-wall | Some larger-thread applications |
Use a proper spark plug socket — it has a rubber insert that grips the plug and protects the ceramic. On modern engines a thin-wall 5/8" reaches down narrow plug wells.
The gap is the tiny distance the spark jumps at the electrode. Too small = weak spark; too big = misfires. It's engine-specific — this is the range, not a substitute for your spec.
| Typical range | Where it lands |
|---|---|
| 0.028"–0.035" 0.7–0.9 mm | Many older / high-boost engines |
| 0.040"–0.044" 1.0–1.1 mm | Most modern coil-on-plug engines |
| 0.050"–0.060" 1.3–1.5 mm | Some late-model wide-gap designs |
5/8" (16 mm) fits most modern plugs — that's the one to have. Older or larger plugs use 13/16" (20.6 mm). Deep, narrow plug wells need a thin-wall spark plug socket. Get one with a rubber insert so it grips the plug and protects the ceramic.
It depends on the engine — commonly 0.040"–0.044" on modern coil-on-plug engines, but anywhere from ~0.028" to 0.060". The exact number is on the underhood sticker or in the manual. Always set to that, not to a generic value.
Check them, yes. Many come "pre-gapped," but the gap can shift in the box, so verify with a gauge before installing. Fine-wire iridium/platinum plugs need a very light touch — adjust the ground strap only, and only a little.
To the torque spec if you can (typically ~13–25 ft-lb depending on thread size and seat). By hand: seat it finger-tight, then about 1/2 turn for a gasket plug or 1/16 turn for a tapered-seat plug. Over-tightening cracks the shell or strips aluminum heads.