The right selection of the coupling thread is very important since it establishes whether the coupling can be coupled while the hydraulic system is under operating pressure. The coupling action is performed by hand, turning the swivel nut by two fingers, possibly without the aid of any tool.
An other requirement is that that test couplers are interchangeable, even if from different suppliers. This to avoid that on large plant or machinery checking various sections would require differing test hose connectors or adapters. The initial design proposal suggested the coupling thread M 16 X 2.
The currently most widely used coupling thread is in fact the M 16 X 2.
The reasons for the initial choice of this coupling thread are not well known, however, it could be that the design of the protection cap has had some influence. A technical explanation results from a detailed analysis based on the principles of mechanics: