Electrode gap
The gap is the small air space at the very tip of the plug where the spark actually jumps. It looks trivial — a fraction of a millimetre — yet it decides whether your engine fires cleanly, fouls, misfires, or quietly cooks an ignition coil. The good news: unlike the heat range, the gap is adjustable. The catch: it is set by the engine maker, not by the plug brand, and getting it wrong has real consequences.
What the gap actually is
The gap is the distance between the centre electrode and the ground electrode (the little metal strap bent over the tip). When the coil delivers its high-voltage pulse, the spark has to bridge that space — and it's the spark, jumping across the gap, that ignites the air/fuel mixture.
Typical values run from about 0.6 mm to 1.1 mm depending on the engine. That sounds like nothing, but the voltage needed to jump the gap rises steeply with distance, so a few tenths of a millimetre change everything.
The correct gap is an engine specification, not a plug specification. The same plug reference can be fitted to several engines, each calling for a different gap. Always take the figure from the vehicle or engine manual (or the under-bonnet sticker), never from the plug box alone.
Too small, too large: why it matters
Gap too small
- The spark is short and weak — a small spark kernel that struggles to light the mixture;
- Incomplete, lazy combustion: rough idle, hesitation, higher consumption;
- The tip fouls more easily because the spark lacks the energy to keep it clean.
Gap too large
- The coil has to generate much higher voltage to bridge the distance — and sometimes it simply can't;
- Result: misfires, hard starting (especially cold or wet), stumbling under load;
- Constant high-voltage demand puts the ignition coil and HT leads under stress — over time this can break down insulation and kill a coil.
This is the key trade-off: a wider gap gives a bigger, hotter spark if the ignition system can feed it — but push it too far and the system can't deliver, so you lose the spark entirely.
Typical gaps by use
| Use case | Typical gap | Notes |
|---|---|---|
| Small engines (mowers, chainsaws, generators) | 0.6 – 0.8 mm | Often a single-strap plug; check the equipment manual. |
| Classic / older car engines (points or early electronic ignition) | 0.7 – 0.9 mm | Weaker ignition energy — keep the gap conservative. |
| Modern road cars (coil-on-plug, high-energy ignition) | 0.9 – 1.1 mm | Strong coils support a wider gap for a fuller spark. |
| Motorcycles, small high-revving engines | 0.6 – 0.8 mm | Tighter gap fires reliably at high rpm. |
| Turbo / boosted engines | Often narrower | High cylinder pressure makes the spark harder to jump — many builders close the gap slightly. |
These are indicative ranges. The figure on your manual always wins.
How to measure it
Use a wire feeler gauge (a set of round wires of known diameter), not a flat blade gauge. A flat blade rests on the high spots and reads larger than the true gap, especially once the electrodes have worn unevenly. With a wire gauge you push the correct-diameter wire through the gap: it should pass with a slight drag, not fall through and not jam.
How to adjust it
Adjustment is done by gently moving the ground electrode (the bent strap) only:
- To close the gap: press the ground strap down gently against a clean surface, or use the bending tool;
- To open the gap: lift the strap slightly with the hook of the gauge tool;
- Move in small steps and re-measure each time.
Never lever against the centre electrode. Prying or pushing on the centre electrode — or on the ceramic insulator around it — can crack the insulator or knock the tip out of true. Always work the ground strap, and never force it.
Iridium and platinum fine-wire plugs are usually pre-gapped at the factory and they are fragile. The precious-metal tip is tiny and brittle. Gently checking the gap is fine; forcing the strap, or letting the gauge catch on the centre tip, can snap the iridium or platinum point and ruin the plug. If the gap is already correct out of the box — leave it alone.
A worn plug is a widening gap
The gap doesn't stay fixed for life. Every spark erodes a little metal from the electrodes, so the gap slowly grows as the plug ages. A gap that has crept well past spec is the clearest sign the plug is simply worn out. Don't keep re-closing it forever — that's a band-aid. Once the electrodes are noticeably eroded, replace the plug.
The common mistake
The classic error is fitting a fresh set of plugs straight out of the box and assuming the gap is right. Concrete example: a workshop fits new copper plugs gapped at 1.0 mm into a small older engine that specifies 0.7 mm. The engine starts hard, misfires under acceleration, and the ignition coil runs hot trying to bridge the oversized gap. Five minutes with a wire gauge to close each plug to 0.7 mm — and it runs perfectly. The plugs were never faulty; the gap was.
In short
- The gap is the spark's jumping distance, typically 0.6–1.1 mm — set by the engine maker, not the plug brand.
- Too small = weak spark, fouling, poor combustion. Too large = misfires, hard starting, coil/ignition stress (and possible coil failure).
- Measure with a wire feeler gauge; it should pass with a slight drag.
- Adjust by moving the ground strap only, gently — never lever against the centre electrode.
- Iridium/platinum plugs are usually pre-gapped and fragile: check, don't force.
- A gap that keeps widening means a worn plug — replace it rather than re-gapping indefinitely.