Shop shortening of thousandth, the basic unit for sizes, offsets and tolerances in inch-based shops. A human hair runs two to four thou, a sheet of copy paper about three to four. Machinists count in thou out loud, as in leave five thou for the finish pass or it's a thou and a half big, and the word mil, used for coatings and sheet, is avoided because it sounds like millimeter.
You're two thou over on the bore, take a spring pass and check it again before you pull it out of the vise.
Plus or minus two thou means 0.002 in each way, so the whole band is four thou wide; rookies often halve it by mistake.
Also heard as
- thousandth
- thous
- mil
Do not mix it up with
- Tenth: A tenth is a ten-thousandth of an inch, ten times smaller than a thou, never a tenth of an inch.
Related terms
Tenth
One ten-thousandth of an inch (0.0001 in, about 2.5 microns), the unit of precision grinding and fits.
Mic
Short for micrometer, pronounced mike, and the verb for measuring a part with one.
Tolerance
The total amount a dimension is allowed to vary from nominal and still be a good part.
Dead nuts
Shop slang for exactly on size or perfectly aligned, with no measurable error left.