Spark Plug Gap Chart: Sizes in Inches & Millimetres

What gap does a spark plug need?

The spark-plug gap is the distance between the centre electrode and the ground electrode — the spark has to jump it. A gap too wide misfires under load, because the coil cannot push the spark across; a gap too narrow gives a weak spark and wastes fuel. One inch is exactly 25.4 millimetres. Typical gaps run 0.024–0.054 inches. Turbocharged and supercharged engines run tighter gaps, 0.024–0.032 inches, because the boosted air can blow the spark out. Modern naturally aspirated engines with platinum or iridium plugs run wider gaps, 0.044–0.054 inches. Always check the owner’s manual or the sticker under the bonnet — the gap is set for the engine, not the plug brand. Gap with a wire-type gauge, never pry on the centre electrode. Diesel engines have glow plugs, not spark plugs, so they have no gap to set.

Two things about the model are worth stating plainly. First, the gap in millimetres is the gap in inches times 25.4 — 0.044 × 25.4 = 1.12 mm. Second, the matching types are every table row with exactly your gap — at 0.044 inches that is two types: early electronic ignition and modern naturally aspirated engines with copper plugs; all listed in Accuracy & limitations.

The spark plug gap chart

Two gaps appear twice in the chart — 0.028 inches is shared by turbocharged engines and motorcycle / ATV engines, and 0.044 inches is shared by early electronic ignition and modern naturally aspirated engines with copper plugs. The tightest standard gap is the 0.024-inch high-boost turbo gap at 0.61 mm, and the widest is the 0.054-inch Ford modular V8 gap at 1.37 mm.

The worked example, line by line

Setup: 0.044-inch gap.

Line 1 — the gap in millimetres. The gap in millimetres is 0.044 × 25.4 = 1.12 mm.

Line 2 — the matching types. The table rows at exactly 0.044 inches are early electronic ignition and modern naturally aspirated engines with copper plugs — two types.

Line 3 — the switch from turbo to modern platinum. Switching from a 0.028-inch turbo gap to a 0.050-inch modern naturally aspirated platinum gap widens the gap by 0.022 inches — 0.56 mm.

Line 4 — the check. 0.044 × 25.4 = 1.12 mm, the gap; and 0.050 − 0.028 = 0.022 inches, the widening.

Every branch, checked

Gap 0.028 in: Gap 0.028 in; 0.71 mm; matching types: Turbocharged, Motorcycle / ATV — At 0.028 inches the gap is 0.71 mm and matches two types — turbocharged and motorcycle / ATV.

Gap 0.030 in: Gap 0.030 in; 0.76 mm; matching types: Small engines (lawn mower, trimmer) — At 0.030 inches the gap is 0.76 mm and matches small engines (lawn mower, trimmer).

Gap 0.035 in: Gap 0.035 in; 0.89 mm; matching types: Classic points ignition — At 0.035 inches the gap is 0.89 mm and matches classic points ignition.

Gap 0.040 in: Gap 0.040 in; 1.02 mm; matching types: Carbureted V8 (1970s–80s) — At 0.040 inches the gap is 1.02 mm and matches the carbureted V8 (1970s–80s).

Gap 0.050 in: Gap 0.050 in; 1.27 mm; matching types: Modern naturally aspirated, platinum/iridium plugs — At 0.050 inches the gap is 1.27 mm and matches modern naturally aspirated engines with platinum/iridium plugs.

Gap 0.054 in: Gap 0.054 in; 1.37 mm; matching types: Ford modular V8 — At 0.054 inches the gap is 1.37 mm and matches the Ford modular V8.

Gap 0.038 in: Gap 0.038 in; 0.97 mm; no exact match — At 0.038 inches the gap is 0.97 mm and matches nothing — it sits between the 0.035-inch classic points gap and the 0.040-inch carbureted V8 gap.

The reverses, worked backwards

Smallest gap (to the thousandth) for the millimetre figure to reach 1.00 mm: At the displayed 0.040 in the figures are Gap 0.040 in; 1.02 mm; matching types: Carbureted V8 (1970s–80s). At the displayed 0.040 in the gap is 1.02 mm. At 0.039 in the gap is 0.99 mm — below 1.00 — so 0.040 in is the smallest gap (to the thousandth) for the millimetre figure to reach 1.00 mm.

Largest gap (to the thousandth) for the millimetre figure to STAY at or below 0.75 mm: At the displayed 0.029 in the figures are Gap 0.029 in; 0.74 mm; no exact match. At the displayed 0.029 in the gap is 0.74 mm. At 0.030 in the gap is 0.76 mm — above 0.75 — so 0.029 in is the largest gap (to the thousandth) for the millimetre figure to stay at or below 0.75 mm.

Smallest gap (to the thousandth) for the millimetre figure to reach 1.25 mm: At the displayed 0.050 in the figures are Gap 0.050 in; 1.27 mm; matching types: Modern naturally aspirated, platinum/iridium plugs. At the displayed 0.050 in the gap is 1.27 mm. At 0.049 in the gap is 1.24 mm — below 1.25 — so 0.050 in is the smallest gap (to the thousandth) for the millimetre figure to reach 1.25 mm.

Smallest gap (to the thousandth) matching turbo: At the displayed 0.028 in the figures are Gap 0.028 in; 0.71 mm; matching types: Turbocharged, Motorcycle / ATV. At the displayed 0.028 in the gap is 0.71 mm and matches Turbocharged, Motorcycle / ATV. At 0.027 in the gap is 0.69 mm — no exact match — so 0.028 in is the smallest gap (to the thousandth) matching turbo.

Largest gap (to the thousandth) for the millimetre figure to STAY at or below 1.10 mm: At the displayed 0.043 in the figures are Gap 0.043 in; 1.09 mm; no exact match. At the displayed 0.043 in the gap is 1.09 mm. At 0.044 in the gap is 1.12 mm — above 1.10 — so 0.043 in is the largest gap (to the thousandth) for the millimetre figure to stay at or below 1.10 mm.

Smallest gap (to the thousandth) for the millimetre figure to reach 0.80 mm: At the displayed 0.032 in the figures are Gap 0.032 in; 0.81 mm; matching types: Supercharged. At the displayed 0.032 in the gap is 0.81 mm. At 0.031 in the gap is 0.79 mm — below 0.80 — so 0.032 in is the smallest gap (to the thousandth) for the millimetre figure to reach 0.80 mm.

What the defaults assume

The defaults are a 0.044-inch gap — Gap 0.044 in; 1.12 mm; matching types: Early electronic ignition, Modern naturally aspirated, copper plugs. With Gap 0.028 in, Gap 0.028 in; 0.71 mm; matching types: Turbocharged, Motorcycle / ATV. With Gap 0.030 in, Gap 0.030 in; 0.76 mm; matching types: Small engines (lawn mower, trimmer). With Gap 0.035 in, Gap 0.035 in; 0.89 mm; matching types: Classic points ignition. With Gap 0.040 in, Gap 0.040 in; 1.02 mm; matching types: Carbureted V8 (1970s–80s). With Gap 0.050 in, Gap 0.050 in; 1.27 mm; matching types: Modern naturally aspirated, platinum/iridium plugs. With Gap 0.054 in, Gap 0.054 in; 1.37 mm; matching types: Ford modular V8. With Gap 0.038 in, Gap 0.038 in; 0.97 mm; no exact match. With Gap 0.000 in, Gap 0.000 in; 0.00 mm; no exact match. With Gap 0.060 in, Gap 0.060 in; 1.52 mm; no exact match. With Gap 0.0445 in, Gap 0.0445 in; 1.13 mm; no exact match. With Gap 0.024 in, Gap 0.024 in; 0.61 mm; matching types: High-boost turbo. With Gap 0.032 in, Gap 0.032 in; 0.81 mm; matching types: Supercharged. With Gap 0.100 in, Gap 0.100 in; 2.54 mm; no exact match. With Gap 1.000 in, Gap 1.000 in; 25.40 mm; no exact match.

A note on the edges: a zero gap means the electrodes touch — a 0.000-inch gap is 0.00 mm and there is no exact match. A 0.060-inch gap is 1.52 mm and matches nothing — wider than any standard gap. A 0.0445-inch gap is 1.13 mm and matches nothing; the nearest standard is 0.044 in. A 1.000-inch gap is 25.40 mm — one inch is exactly 25.4 mm — and matches nothing. A negative gap input is floored at 0 — it shows 0.00 mm and no matching type, never a negative number.

Output · formula · meaning

Who uses this

• The DIY mechanic gapping plugs for a turbo: they learn a turbocharged engine takes a 0.028-inch gap — 0.71 mm — and they check the 0.028 in reverse to see the smallest gap that matches turbo.
• The owner of a modern V8: they learn a Ford modular V8 takes a 0.054-inch gap — 1.37 mm — and they check the 0.050 in reverse to see the smallest gap that reaches 1.25 mm.
• The owner of a lawn mower: they learn a small engine takes a 0.030-inch gap — 0.76 mm — and they check the 0.029 in reverse to see the largest gap that stays at or below 0.75 mm.
• The owner with a 0.038-inch reading: they learn it is 0.97 mm and matches no standard type — and they check the 0.043 in reverse to see the largest gap that stays at or below 1.10 mm.

Accuracy & limitations

The math is first-principles: the gap in millimetres is the gap in inches times 25.4, rounded to two decimals half-up; the matching types are the table rows with exactly your gap; the gap in inches is floored at 0 — a negative gap input shows 0.00 mm and no matching type, never a negative number. The reverses are judged on the UNROUNDED millimetre figure: at 0.039 in the gap is 0.99 mm, below the 1.00 mm target, so it does not meet the bar. What the model cannot do is set your gap: the gaps in the table are the typical figures, not a rule for every engine — always check the owner’s manual or the sticker under the bonnet, because the gap is set for the engine, not the plug brand; a gap too wide misfires under load and a gap too narrow gives a weak spark and wastes fuel. Use it to find the millimetre figure and the matching types for your gap — then gap with a wire-type gauge, never pry on the centre electrode.

Frequently asked questions