Metric Threads Explained

The size of a metric fastener (screw, bolt, etc.) is generally specified as diameter, pitch and length, all in millimeters (mm). Metric fasteners are usually classified as either coarse or fine, referring to their thread pitch, although some sizes are available in an extra fine thread. Coarse is the most common metric thread and is assumed when pitch isn't specified.

Consider this size…

M8-1.0 x 20

Here's what the description means…

M = Metric thread designation

8 = Nominal diameter, in millimeters

1.0 = Pitch (distance from thread to thread), in millimeters

20 = Length, in millimeters

Remember these tips about metric fasteners…

Diameter: the larger the number, the larger the screw or bolt

Pitch: the larger the number, the more coarse the thread (the fewer the threads per unit of measure); coarse is the most common metric thread and is assumed when pitch isn't specified

Length: the larger the number, the longer the screw or bolt

Standard: the DIN or ISO standard number describes the fastener

What size Drill bit do I use to tap M...?

Tip: To calculate the tap drill size for metric threads the diameter of thread minus the thread pitch will give you the required tap drill size. So for M6 x 1 thread size. 6 - 1 = 5mm. Use a 5mm drill.