There are more than 100 different types of thread, from ACME, an American trapezoidal thread, to Yv, a Turkish round thread. This large variety is due to historical and cultural reasons. Many countries and industries have created their own standards. Aviation, railways, as well as the Swiss watch industry
work according to their own rules. We would like to briefly present the most common types which are used in piping technology:

Cylindrical / Parallel threads (G)

The diameter of the thread is constant over the entire length. It can be screwed into a cylindrical counterpart without force until it reaches a stop. This type of thread is suitable for fastening. It does not have a sealing function. For example, all SERTO threads between the union nut and the body of the union have a cylindrical form. They firmly clamp the compression ferrule and therefore the tube against the body of the union. An additional seal is necessary if the cylindrical thread is to be used as a male thread. This is implemented either with a metal seal edge (DIN 3852-2, Form B and Form A with additional sealing washer) or by means of a soft seal.

The advantage of this thread is the sealing method: Due to the stop, the installed union has a defined screw-in depth, which does not change even after repeated assembly. The seal itself can withstand several assemblies. However, good quality material and a flat sealing surface are essential for a good seal.

Tapered / Conical threads (R)

The diameter of the thread increases with each turn of the thread. The torque increases if it is screwed into a cylindrical thread. In
theory, this combination seals by compressing and wedging the two parts of the fitting. Due to manufacturing tolerances, the sealing
effect is only achieved by means of a sealing tape (PTFE tape) or the application of a sealing agent. Due to the less demanding manufacturing and quality requirements, tapered threads are cheaper than cylindrical threads. In use, and especially in the case of
repeated assembly, a weak point becomes apparent: the sealing agent needs to be re-applied for each assembly. Due to the fact that the thickness of the sealant can never be precisely determined, the installation depth of the union can vary. Pre-assembled tubes may be too long or too short. For moulded components such as elbow unions or T-unions, the conical thread is an advantage, because
the alignment of the component can be adjusted.

• The BSP (British Standard Pipe) thread (G) is also known as a Whitworth pipe thread, after its inventor. The diameter of the thread is stated in inches, the flange angle is 55° and is the most common thread which is used in Europe and by SERTO. In the BSPT version, the thread is tapered. At SERTO the thread sizes are stated on the hexagons. (e.g.: 1/8)
• The American NPT (National Pipe Thread) pipe thread is a tapered male/female thread, which is defined in the standard ANSI B 1.20.1. The external diameter is stated in inches. The thread size is supplemented with NPT on SERTO hexagons. (e.g.: 1/8 NPT)
• Metric ISO thread (Mz), also known as standard thread is a globally standardised thread. Thread diameters are stated in mm. The marking on SERTO hexagons is e.g.: M8x1.
   

Extract from Uptodate 41