Hydraulic Fittings

Author: CEmarking TEAM | Last edited: 16.06.2020

confusion between metric and imperial threads

A closer look at the hydraulic screw connections shows that most of the screw connections commonly used in hydraulic systems are not marked by the manufacturer with regard to their thread type. In addition, some thread forms are very difficult to distinguish due to their external geometric similarity and size. Some screw-in studs and screw-in holes of different thread types fit together, although they are dangerous, non-pressure connections.
For example, screw connections with an inch G 1⁄2" thread can be screwed into metric M 22x1.5 holes almost unnoticed and without problems, without overtightening one of the threads. However, this faulty connection, consisting e.g. of an imperial screw-in stud in a metric screw-in hole, cannot withstand the operating pressures and can break away abruptly. This can lead to considerable injuries due to flying parts and escaping hydraulic fluid.

Current standards EN ISO 4413 specifies in clause

Within a hydraulic system, ports, studs, and connectors should be limited to the minimum number of standard series possible. All port connections should be in accordance with ISO 6149-1, ISO 6149-2 and ISO 6149-3 for threaded port connections, or ISO 6162-1, ISO 6162-2 or ISO 6164 for four-screw flange port connections.

It is furthermore noted in DIN EN ISO 4413

When using more than one type of ISO standard threaded port connection (e. g. ISO 1179, ISO 9974 and ISO 11926) in a system, there is a strong possibility of incompatible intermixing between some stud ends from one connection series and some ports from a different connection series, causing leakage and severe failure of the connection system. Ports and stud ends that conform to ISO 6149-1, ISO 6149-2 and ISO 6149-3 are marked such that they are identifiable.


In order to avoid confusion of the threads, it is generally recommended that the metric screw-in holes and studs according to ISO 9974 and the imperial ones according to ISO 1179 are also marked in analogy to those of ISO 6149 part 1-3 (screw-in holes and studs with thread according to ISO 261 and O-ring sealing). This is a task for the manufacturers of bolting systems and their components. The standardization committees are requested to to include requirements regarding the marking of screw-in studs and screw-in holes in the revisions or updates of the relevant standards. The designers of the machine manufacturers, technical buyers and also operators, operational planners and maintenance staff should ask for marked screw-in holes and screw-in pins for the machines.

Recommended measures

The danger of possible mix-ups can be reduced by changing over to screw connections of a standard series on a machine.

In future, all screw-in holes and studs with metric threads according to ISO 261 should be marked with a circumferential notch (screw-in stud) and with a raised ring line or a stamped "M" (screw-in hole) (see EN ISO 6149 Parts 1 - 3). This would clearly identify the metric threads. In future, the inch threads according to EN ISO 228 should be marked in a different way so that the distinction to metric threads is clear.


Since experience has shown that revisions of standards and the subsequent transition periods extend over longer periods, it is recommended that such markings should already be prescribed in the factory standards today.

In particular, it is recommended to convert end customer specifications to a series of standards. The specifications made on thread form and its marking can be included in company work instructions.