How-To
How to Gap a Spark Plug Correctly
Gap is the distance the spark has to jump. Too wide and the spark may not form; too narrow and it's weak. Setting it takes two minutes and a $3 tool.
What you need
- A gap gauge (coin-style or wire-style — wire is more accurate for fine gaps)
- Your engine's specified gap (owner's manual, or the part-number decoder)
Steps
- Find the spec. Look up the exact gap for your engine. It's usually between 0.028" and 0.060".
- Measure the current gap. Slide the correct-size gauge between the center and side electrode. It should pass with light drag.
- Adjust if needed. To close the gap, gently tap the side electrode on a hard surface. To widen it, hook the gauge under the side electrode and lever outward — a hair at a time.
- Re-measure. Check again and repeat until the gauge drags lightly through.
- Never pry against the center electrode. On iridium and platinum plugs the fine center tip is fragile — only ever move the side (ground) electrode.
Many modern iridium plugs come pre-gapped from the factory and should not be adjusted at all unless the spec differs. Check before you touch them.
Why gap matters
The ignition coil has to generate enough voltage to bridge the gap. A worn plug's gap grows as the electrode erodes, demanding more and more voltage until the coil can't keep up — and you get a misfire. That's why gap drifts out of spec as plugs age, and why replacing them restores smooth running.