Metric Hydraulic Fittings: DIN, JIS, and Komatsu Basics

Metric hydraulic fittings are not one single connection style. In field maintenance, the term often includes DIN, JIS, Komatsu, BSPP-related connections, and other regional standards that may look similar but seal in different ways.

The safest approach is to identify the standard, thread pitch, sealing angle, and port style before ordering a replacement.

DIN Metric Fittings

DIN metric fitting standards cover hydraulic hose fittings and compression fittings. DIN and BSPP are common thread families in Europe, while BSPP is also common in Australia and other Commonwealth markets.

DIN 2353 is one of the most widely used bite-type tube fitting systems. It uses a 24 degree cone connection and does not depend on an O-ring to create the main bite-type tube seal. ISO 8434 style metric tube fittings also use 24 degree geometry in standard compression fitting work.

Other DIN metric styles use different sealing forms. DIN 20078 series hose fittings also use a 24 degree cone. DIN 7631 uses a 60 degree cone in hydraulic systems, while DIN 74233 and DIN 74234 use 45 degree cone styles.

JIS Metric Fittings

JIS metric fittings are common as hose adapters on equipment manufactured in Japan and Korea. They may incorporate 30 degree flare and 60 degree cone connections and often include BSP thread forms.

JIS B2351 and metric ISO 6149 port connections both use elastomeric O-ring sealing at the base of the thread. This kind of O-ring port seal can help with vibration resistance in construction equipment and frequently actuated industrial machinery.

Komatsu and Other OEM Metric Styles

Komatsu metric fittings are a specialized OEM style. The Komatsu metric 30 degree flare is common on certain mobile equipment. It should not be replaced by another metric fitting only because the thread diameter appears close.

Kobelco-related French G GAZ connections use metric straight threads with a 24 degree sealing surface. They can resemble German DIN fittings, but the thread details are not always the same. Fine-thread and standard-thread differences matter when the connection is tightened.

Parallel and Tapered Metric Threads

Metric fittings can use either parallel or tapered threads. A parallel metric thread in a port application usually seals with an O-ring or other seal at the top or base of the thread. A tapered metric thread seals through metal-to-metal wedging of the threads.

Do not assume a parallel thread can replace a tapered thread, or that an O-ring port can replace a cone-seat connection. The sealing method is as important as the pitch.

How to Identify Metric Threads

1. Check whether the thread is tapered or parallel.
2. Measure the thread pitch in millimeters with calipers or a thread gauge, such as 1.0, 1.5, or 2.0 mm.
3. Measure the male thread outside diameter or the female thread inside diameter.
4. Match the measurement to the closest metric thread callout. For example, M8 x 1 means an 8 mm outside diameter with a 1 mm pitch.

Why Metric Threads Are Practical

In maintenance, the choice between metric and inch-series fittings is usually already made by the OEM. You must use the connection the machine was designed around.

Metric threads have practical advantages: the measurement system is based on tenths, common sizes can simplify bills of material, and metric connections are widely used in global equipment. Metric thread forms can also provide good vibration resistance and a larger stress area in some threaded cross sections.

When replacing DIN, JIS, Komatsu, BSPP, or ISO metric fittings, record the thread pitch, sealing angle, O-ring location, tube size, and equipment make before ordering.