A number 6 thread typically refers to one of two common standards, depending on whether the context is hardware (screws/bolts) or sewing supplies (yarn/thread):
No. 6 (3.5mm) Thread Size. 6.8mm Head Diameter (D) 2.9mm Pilot Hole Size.
M6 Threads
M6 refers to a metric 6 mm screw. The outside diameter of the threads is 6 mm.
For example, if you see a screw labeled M6 , that simply means 6 mm in diameter. How to use this chart: Find your metric size in the left column, then grab the decimal inch and the closest common fraction most mechanics use.
No. 6 (3.5mm) Thread Size.
A gage number like "6" is the diameter and the length is the length of the screw that goes into the wood not inclusive of the head unless it's a flat head. The smaller the gage number the smaller the screw diameter (opposite of nails and wire diameter).
The tap drill for an M6 thread is 5 mm. The clearance drill for an M6 thread is 6.6 mm.
M6 and M8 are close in size, but are not interchangeable with 1/4" and 5/16". The thread pitch is different, so it won't thread into the same hole even if the diameter is roughly the same.
The basic rule is: Lower numbers mean thicker threads. Higher numbers mean finer threads. One common way to measure thread weight is the 'metric count' system, which is based on the metric system. In this system, the metric count (Nm) indicates how many 1000-meter hanks of thread weigh one kilogram.
The main difference between 10-32 and 12-24 threads is their dimensions. 10-32 screws have a diameter of 3/16″ (4.8mm) where as 12-24 screws have a diameter of 7/32″ (5.6mm). M6 threads are 6mm in diameter, only .
The 505 screw is a #6-32 screw with a major thread diameter of #6, which is defined as 3.51 mm (0.138 in.).
5mm = just over 3/16 inch. 6mm = almost 1/4 inch. 7mm = almost 9/32 inch (= a bit over 1/4 inch)
Imagine holding a standard paperclip; it's about 1 mm thick, so six millimeters would stack up to about six paperclips lined up side by side.
Metric Thread Size Explained
The most common system adhered to is the ISO Metric Screw Thread, where designations like M6x1 signify a M6 screw with major diameter of 6 mm and a pitch of 1 mm.