ISO Connections and Instrumentation Fitting Basics

ISO hydraulic connections and instrumentation fittings cover several different connection families. Three common groups are ISO 6162 four-bolt flange connections, ISO 6149 metric O-ring ports, and single or double ferrule instrumentation fittings.

Each group seals differently. One may use an O-ring compressed at a flange face, another uses an O-ring at a metric straight-thread port, and another uses ferrules gripping a tube.

ISO 6162 four-bolt flanges

ISO 6162 four-bolt flanges use a rectangular four-bolt pattern and a flanged head with an O-ring groove. The O-ring is compressed between the flanged head and the flat port face while bolts hold the connection together.

There are two common pressure groups. Code 61 is the standard-pressure series, often associated with PN 35 or 350 bar references. Code 62 is the high-pressure series, often associated with PN 415 bar references. The two styles look similar, but Code 62 uses larger bolt spacing and larger flanged head dimensions.

The female port is smooth and unthreaded at the flow opening, with the rectangular bolt pattern around it. The male side uses the flanged head, O-ring, and split or captive flange halves. Inch or metric bolts may be used, and metric versions may be marked with an M.

The seal is made where the O-ring is compressed between the flanged head and the flat surface of the port. The threaded bolts hold the flange halves in place, but the O-ring and flat port face create the seal.

ISO 6149 metric O-ring ports

ISO 6149 ports use ISO metric straight threads with an O-ring seal. The layout is similar in concept to SAE O-ring boss ports, but the thread form is metric.

The male connector has straight metric threads and an O-ring. The female port has straight threads, a spotface, and a chamfer where the O-ring seals. The threads provide mechanical retention, while the O-ring creates the seal.

Because the seal is made by the O-ring and machined port geometry, a damaged chamfer, wrong O-ring, or mismatched metric thread can make the connection leak even when the fitting appears close.

Double ferrule instrumentation fittings

Double ferrule instrumentation fittings use a body, nut, front ferrule, and back ferrule. The tube enters a counter-bore in the fitting body. As the nut is tightened, the ferrules grip the tube and create the seal.

These fittings are used in fluid and gas applications such as refineries, chemical plants, and food-processing systems. The front ferrule seals, while the back ferrule helps grip the tube.

Double ferrule fittings allow unflared tubing to be used. The grip comes from the swaging action created when the nut forces the front and back ferrules into the tube and fitting body.

Single ferrule instrumentation fittings

Single ferrule fittings use one larger ferrule rather than a front and back ferrule pair. They serve a similar purpose but grip and seal the tube differently.

Instrumentation fittings are commonly available in stainless steel and brass. They often use UNEF extra-fine threads, and sizing is usually based on tube outside diameter rather than pipe size. That is why tube OD, ferrule style, thread form, and sealing method have to be identified together.

Code 61 and Code 62 should not be mixed

The ISO 6162 section needs more room because the two flange codes can look similar in a photo. Code 61 and Code 62 both use a four-bolt layout with an O-ring at the flanged head, but the bolt spacing and head dimensions are different. Code 62 is the higher-pressure series, so a near match is not enough.

The port may be smooth and unthreaded at the flow opening, while the bolts provide the clamp load. That means the seal depends on the O-ring groove, flat port face, correct flange halves, bolt length, and tightening practice. A damaged groove or mismatched bolt pattern can leak even when the fitting appears to sit in place.

Metric O-ring ports and ferrule fittings

ISO 6149 also deserves separate identification because it resembles SAE ORB in concept but uses metric threads. The spotface and chamfer have to be clean, and the O-ring has to compress in the right location.

For instrumentation fittings, double ferrule and single ferrule designs both use unflared tubing, but they grip the tube differently. Tube OD, tube hardness, ferrule count, nut thread, body material, and whether the line carries gas or liquid should all be recorded before substituting parts.