A fastener length is measured from where the material surface is
assumed to be, to the end of the fastener.
For fasteners where the head usually sits above the
surface, the measurement is from directly under the head to the end of
the fastener. Our button head and socket head cap screws are measured in this way.
For fasteners that are designed to be countersunk, the
measurement is made from the point on the head where the surface of the
material is, to the end of the fastener. Our flat head screws are measured in this way.
US Fasteners
US Fasteners are described by the diameter, thread count (machine threads
only) and length.
The diameter and length are in inches with the exception of small screws
whose diameter may be a size number (ex. 12). In this case the number will
often have a # sign before it (ex. #12)
Example:
1/4-20 x 6
This has a diameter of 1/4 inch, 20 threads per inch (course thread) and
a length of 6 inches.
Metric Fasteners
Metric fasteners are described in a similar manner to US fasteners.
However, instead of threads per inch, the metric system uses thread pitch
(the length of each thread in millimeters measured along the shaft of the
bolt). Typically the diameter of a metric bolt is preceded by a capital M to
designate metric.
Example:
M10 x 1.5 x 20
This has a diameter of 10mm, a 1.5 thread pitch and measures 20mm long.
