JIS B 2351 Fitting Sizes: Japanese Metric Thread Data

JIS B 2351 uses Metric threads with a metal-to-metal seal per JIS B 2351. Thread dimension table and identification tips below.

Thread Sizes

Note: JIS B 2351 covers two thread variants: metric threads (common on Komatsu) and BSPP threads (common on Hitachi, Kobelco, Sumitomo). This table shows the metric thread variant. If the fitting has BSP parallel threads, see the BSP standard page.

JIS fittings are common on Japanese-built hydraulic equipment. They use a 30-degree flare seat with metric threads. The 30-degree angle distinguishes them from JIC (37-degree) and DIN (24-degree). They look similar to JIC at a glance, but the flare angles are different enough that cross-connecting causes leaks. Always verify the cone angle when working on Japanese equipment.

When to Use JIS

Japanese-built construction equipment (Komatsu, Kobelco, Hitachi, Sumitomo), Japanese industrial machinery.

If the nameplate says it was made in Japan, check for JIS threads before ordering JIC replacements.

Where JIS Falls Short

Easily confused with JIC. The 30-degree vs 37-degree flare difference is subtle but causes leaks if mixed.

Metric threads are standard, which complicates sourcing in imperial-thread markets.

Fewer off-the-shelf options at North American hydraulic suppliers.

How to Identify JIS Fittings

30-degree flare angle with either metric or BSPP threads. Found almost exclusively on Japanese-origin equipment. Komatsu typically uses metric threads; Hitachi, Kobelco, and Sumitomo often use BSPP threads. If the machine was built by a Japanese manufacturer, check for JIS before assuming JIC.

Measurement Tips

JIS fittings use a 30-degree flare angle, compared to 37 degrees for JIC. This smaller angle means the flare is wider relative to the tube. JIS fittings on Japanese equipment are almost always metric threads with metric tube OD. If you are replacing a fitting on a Komatsu, Kobelco, or Hitachi machine and it looks like a JIC but the threads do not quite match, it is probably JIS.

What JIS Gets Confused With

JIS B 2351 covers the 30-degree flare fittings, but Japanese equipment also uses other fitting types including BSP ports and metric O-ring boss ports. Not every fitting on Japanese equipment is JIS. The easiest way to identify JIS specifically is the combination of a 30-degree flare with metric threads. If the threads are metric but the seat angle is flat, it is likely a metric O-ring boss (not JIS flare).

Data sourced from JIS B 2351. Thread dimensions are nominal values. Always verify with a thread identification gauge before making connections.