Spark Plug Gap Converter

How it is calculated

You enter the spark plug gap in millimetres and the tool gives it in inches and in thousandths of an inch (called thou or mils): in = mm ÷ 25.4 and thou = in × 1000. Inches are shown with four decimals and thou are rounded to a whole number. The conversion works in one direction only, mm to inches. Zero is accepted, negative values give no result. The tool does not tell you which gap your engine needs: that value comes from the engine or plug maker's specification. Since thou is rounded to whole numbers, a gap of 0.8 mm shows as 31 thou even though 31.5 is the exact figure, so check which gauge size you have on your feeler set.

Common mistakes

• Entering the gap in inches when the field expects millimetres; a value like 0.031 would be read as 0.031 mm and give about 0.0012 in.
• Treating the rounded thou figure as exact: the whole number hides the half thou (0.8 mm is 31.5 thou, shown as 31), which can matter for a tight spec.
• Reading thou as a decimal inch: 31 thou is 0.031 in, not 31 in or 0.31 in.
• Setting the gap from this conversion without checking the engine spec; a wrong gap in either unit still causes misfires.
• Gapping a new plug by bending the centre electrode instead of the ground electrode, which can damage the plug, whatever the unit.