Can anyone out there explain to me how to measure a bolt and give it's correct technical name, so that when I phone up to order some I don't sound a complete tit - and that i actually end up with what I need.

Thanks - Boner.

if it takes an allen key, then its generally a HEX SOCKET CAP SCREW, it is measured by thread dia i.e 8mm dia = M8, and its length from its shoulder.

for example: ask for M8 x 25L hex socket cap screw, most screws and bolts are measured in the same way apart from countersunk screws which are measured by total length.

also you may want to state material i.e 12.9 high tensile or zinc plated or stainless steel.

also see here

Bolts are normally given in size (Diameter x pitch) and length. I measure the diameter, to get the first, then use a pitch gage on the threads for the second. If no pitch gage is available, you can carefully measure the distance between two thread crests. Here in the states we have to juggle between SAE and metric so you must be carefull when someone hands you a bolt and they don't know the size.

The only other thing I would add is that a bolt is not threaded all the way to the head - it had an elongated shoulder for load bearing purposes. A machine screw or set screw or sometimes machine bolt (all the same) look like a bolt, you'd call it a bolt, but it IS threaded all the way to the head......

So, if you want it threaded all the way, its called a machine screw. If you do not want it threaded all the way, its called a bolt.

Then the choice of head which has been mentioned already. Socket cap is the posh name for allen key head, hex head is your traditional bolt head, button head is like a flattened dome or panhead and csk is countersunk.

Hope that helps

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