Spark Plug Gap Chart
Typical spark plug gap ranges by ignition system type.
| Ignition / plug type | Typical gap |
|---|---|
| Older domestic V8 (points ignition) | 0.028–0.032 in |
| Standard electronic ignition (most 1980s-2000s) | 0.035–0.045 in |
| Modern coil-on-plug, copper/platinum | 0.040–0.045 in |
| Iridium/long-life plugs | 0.030–0.032 in (often pre-set, don't adjust) |
| Boosted/high-compression applications | Often gapped tighter than stock -- check engine builder spec |
Always check your specific application first
These are general ranges by system type, not a substitute for your vehicle or engine's actual specification -- gap requirements vary by manufacturer, ignition coil output, and compression ratio. A gap that's too wide can cause misfires under load (the spark can't reliably jump the gap at high cylinder pressure); too narrow can limit spark energy and cause fouling.
Where these numbers come from
General ranges reflecting typical published specifications across common ignition system generations. Always use the exact gap specified for your vehicle, engine, or spark plug manufacturer over a general range -- especially for boosted or high-compression engines, where the correct gap is often tighter than a stock naturally-aspirated spec.
Frequently asked questions
Why does boost or high compression need a tighter gap?
Higher cylinder pressure makes it harder for the spark to jump the gap reliably -- a tighter gap needs less voltage to fire consistently under those higher-pressure conditions, reducing misfire risk.
Can I adjust the gap on iridium or platinum plugs?
Many are pre-set and not designed to be adjusted -- check the plug manufacturer's instructions, since bending the fine iridium electrode can damage it.
What happens if the gap is too wide?
The spark may not reliably jump the gap under load or at higher RPM, causing misfires -- this gets worse under boost or high cylinder pressure specifically.
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