M numbers nuts and bolts

[Independence]

M10 , M8 etc.

Would someone mind explaining what the 'M' gauge refers to on bolts and screws. I've been buying these for years by sight or comparing to what I am replacing.

As I'm reviewing using mail order via a recommended site it's about time I started using the right lingo.

 

[sailorman]

M=Metric init

 

[Norman_E]

Metric screw threads are standardised, and the metric coarse thread series in "First Choice" diameters are in the most common use. The table in this web page gives the available diameters and thread pitches.

Note that although most modern equipment uses metric fasteners, there are lots of Imperial threads still in use, including UNC, UNF, BSW, BSF and BA (The latter common in older electrical equipment) as well as special purpose threads like BSP (British Standard Pipe, found on gas fittings). In my workshop I have taps and dies in all of the above, plus metric coarse and specialist Model Engineers 32TPI and 40TPI threads. The moral is, do not just assume that any thread you come across is metric.

 

[VicS]

The above links put all the detail on the syetem but all you really need to know is that the number 8, 10 or whatever is the nominal diameter over the thread.

You'll see there are coarse and fine threads. The coarse threads are the ones you'll normally see.