Dimensional Specifications
DIN 931 & DIN 933
The drawing below shows unthreaded fasteners. The thread length of Metric bolts manufactured to DIN 931 and 933, with diameter d and thread length t, is:
t = 2d + 6mm up to 125mm; t = 2d + 12mm from 125mm until 200mm; 2d + 25mm at 200mm and over.
BS 1083
The drawing below shows unthreaded fasteners. The thread length of BSW or BSF bolts manufactured to BS1083, with diameter d and thread length t, is t = 2d.
BS 1768
The drawing below shows unthreaded fasteners. The thread length of UNC or UNF bolts manufactured to BS1768, with diameter d and thread length t, is: t = 2d + 1/4" up to a length of 6", where the formula becomes t = 2d + 1/2".
How to measure
Measuring bolt diameter
Bolts can be accurately measured using a vernier. For metric bolts, the prefix 'M' is added to denote Metric. For example, a 4mm diameter bolt is commonly known as an 'M4' bolt. Imperial bolts are measured in inches.
Measuring bolt length
The length of a Hexagon bolt is determined by measuring from underneath the head to the end of the bolt. If the bolt has a countersunk head, the full length of the bolt including head is measured.
Measuring threads
Metric bolts are identified by measuring the distance between the threads - a 1mm pitch bolt has 1mm between the threads. The thread pitch of an imperial bolt is denoted by the amount of threads per inch. See the thread charts on this page for more information. A thread gauge can be used to identify bolts and nuts, although they are not in common usage. The Thread Detective is a quick and easy solution to the problem of identifying bolt and nut threadforms, allowing quick and precise identification of Metric, UNC & UNF bolts and nuts.
