Also known as a 35 thou gap.
Spark Plug Gap Converter
Convert a spark plug gap between inches, millimeters, and thou. Enter the spec from a manual or tap a common preset to see the matching feeler gauge size.
Convert a spark plug gap
Results update instantly with inch, metric, and thousandths-of-an-inch readings.
A 0.035 inch spark plug gap is 0.889 mm, or 35 thou.
How spark plug gaps are written
Many U.S. manuals list spark plug gaps as decimal inches, such as 0.030 in, 0.035 in, or 0.044 in. Metric specs usually use millimeters, such as 0.8 mm, 0.9 mm, or 1.1 mm.
- 0.030 inch = 0.762 mm = 30 thou
- 0.035 inch = 0.889 mm = 35 thou
- 1.0 mm ≈ 0.0394 inch ≈ 39.4 thou
Use the vehicle spec first
This tool converts the number; it does not choose the correct gap for a specific vehicle. Modern plugs may be pre-gapped, and some fine-wire plugs can be damaged by rough adjustment.
Check the under-hood label, service manual, plug catalog, or engine manufacturer spec before changing the gap.
Common spark plug gap conversion chart
| Inches | Millimeters | Thou | Typical reading |
|---|
Spark plug gap conversion FAQ
What is 0.035 spark plug gap in mm?
0.035 inch is 0.889 mm, commonly rounded to 0.89 mm.
What is 0.044 spark plug gap in mm?
0.044 inch is 1.118 mm, commonly rounded to 1.12 mm.
Is thou the same as thousandths of an inch?
Yes. A 30 thou gap is 0.030 inch, and a 40 thou gap is 0.040 inch.
Can I use a feeler gauge for spark plug gaps?
Yes, a clean wire or blade-style spark plug gap tool can be used. Match the converted size to the nearest gauge and avoid forcing the electrode.