Why are some bolts called metric bolts, some American standard bolts, and others British standard (BS) bolts? The key difference lies in their thread specifications. The earliest invented thread was the British standard thread, featuring a 55-degree triangular thread profile that spirals upward continuously. Early threads were all right-hand threads-tightening requires turning clockwise, which is what we now refer to as standard threads. Left-hand threads, by contrast, were developed later to meet specific application requirements.
The advent of BS bolt threads played a pivotal role in advancing the industrialization of the European market in the early days. The British standard bolt thread is designated by the code BS. In China, its marking format adopts a fractional structure, including two core parameters: the nominal thread diameter and the number of threads per inch (25.4 millimeters). For example, the marking 1/4-20 indicates a nominal thread diameter of 1/4 inch (equivalent to 6.35 millimeters after conversion), and the number 20 refers to the thread count, meaning there are 20 threads distributed over a length of one inch (25.4 millimeters).
Since the parameters of standard BS bolts are well understood, we have compiled the specifications for less commonly used fine-pitch BS bolts, which can be found in the table above. The thread parameters of fine-pitch BS bolts differ from those of standard-pitch bolts, as they adhere to dedicated fine-pitch standards. The term "fine-pitch" means that there are more threads per inch (25.4 millimeters) compared to standard-pitch threads. Of course, the number of threads for fine-pitch specifications is not arbitrarily determined but follows clear standard regulations. For easy differentiation, we have also listed the thread parameters of standard-pitch BS bolts, as shown in the figure below:
Let us take the 1/4-inch BS bolt as an example. The standard-pitch version has 20 threads per inch, while the 3/8-inch standard-pitch bolt has 16 threads per inch. Fine-pitch variants are different: the 3/8-inch fine-pitch BS thread comes in two specifications, with 24 threads per inch and 32 threads per inch respectively.
In fact, the most critical point in this table is not the distinction between coarse and fine pitches. After all, if the thread specifications do not match, the bolt simply cannot be screwed in, allowing the problem to be identified quickly during installation. The real key point to focus on is the 1/2-inch diameter specification: the standard-pitch BS thread has 12 threads per inch, whereas the American standard thread has 13 threads per inch. With only one thread difference per inch (25.4 millimeters), this discrepancy is almost impossible to distinguish with the naked eye. Worse still, during installation, a 12-thread-per-inch BS bolt can actually be screwed into a 13-thread-per-inch American standard threaded hole-only to become increasingly stuck as it is turned further. By the time the mismatch is discovered, thread damage has often already occurred.
Therefore, when purchasing 1/2-inch diameter bolts, it is essential to clearly specify whether you require the 12-thread-per-inch British standard or the 13-thread-per-inch American standard.
