Question…. When is a bolt not a bolt?
Answer…. When it is a Set Screw.

We are reffering to hexagon headed bolts and set screws – see diagram.
The difference between a bolt and a set screw is that there is thread the entire length of the set screw, whereas a bolt is only partly threaded.

measure
boltvssetscrew
THREAD LENGTHS FOR STANDARD BOLTS
THREAD LENGTH
METRICUp to 125mm long2 x diameter + 6mm
125mm – 200mm long2 x daimeter + 12mm
Over 200mm long2 x daimeter + 25mm
UNC/UNFUp to 6" long2 x dia + 1/4"
Over 6" long2 x dia + 1/2"
BSW/BSFUp to 1/2" dia.Over to 1/2" dia.
Up to 4" long2 x diameter1.1/2 x diameter
4" – 8" long2 x diameter2 x diameter
Over 8" diameter2.1/2 x diameter2.1/2 x diameter

Terminology
In the UK we refer to these products as hexagon headed bolts and setscrews, however, in the USA they refer to them as cap screws.
Another confusing terminology associated with these is the fact that a grub srew is also reffered to as a set screw….confusing isn’t it?

Sizes
As mentioned, a set screw is fully threaed along the length of the body, therefore the length is stated by the measurement from under the head to the end.
The length of a bolt is also stated by the measurement from under the head to the end, but the length of the thread is determined by the diameter, as shown above.

Examples: M10 x 100 bolt = 36mm thread or 1/4″UNC x 2″ bolt = 3/4″ thread or 1/2″BSW x 5″ bolt = 1″ thread.
The plain section (known as the shank) is the remainder of the length.

Note: These sizes are based on the DIN and British Standard specifications for standard bolts, however some manufacturers choose to add an extra turn or two of thread which increase the overall thread length and shortens the resulting plain shank section. There are also less popular hexagon bolts with differing specifications.