Hydraulic fittings: the practical handbook

Four reusable tube connections dominate hydraulics, and all four are gathered in the standard family DIN EN ISO 8434: the 24-degree cutting ring (part 1, also the older DIN 2353), the 37-degree flare or JIC (part 2), the O-ring face seal ORFS (part 3) and the 60-degree cone connection (part 6). Each solves the same task, connecting a tube leak-tight and reusably, but each takes a different route.

The cutting ring bites into the unformed tube as the union nut is tightened and seals metal-to-metal on a 24-degree internal cone; the tube is only cut to length and deburred, not formed. As it is tightened, the cutting edge also creates a spring effect that absorbs pressure surges and bending cycles. The 37-degree flare forms the tube end itself into a cone that seats metallically against the fitting, and needs a flaring tool for it. ORFS seals not in the thread but over an elastomer O-ring in a flat face, which makes the connection especially vibration-resistant and suitable for pressures up to about 420 bar. The 60-degree cone connection seals over a cone seat and shows up mainly on BSP connections.

In the end the sealing principle decides, not the thread alone: the same thread can seal over a cone, a flat face or a seal ring, and anyone who only compares the thread diameter quickly gets it wrong. The O-ring face seal is covered in detail by the article ORFS fittings, and the fundamental choice between formed and unformed tube by the article flare or cutting ring. What matters in practice: the four systems are not interchangeable. A 24-degree cutting-ring body does not accept a 37-degree flared tube; if you switch, you replace the whole connection, not just one part.

Two variants share the 24-degree cone. The pure cutting-ring version seals metal-to-metal between ring, cone and tube; the DKO version additionally places an O-ring in the seal cone and thus seals elastomerically and metallically at once, which makes it less sensitive to settling and loosening. Both share series, thread and rated pressure, so you choose between them within a series without changing the sizing. All four designs also come in stainless steel and for aggressive media; the material choice follows the medium and environment, not the sealing principle. For the ORFS O-ring that means NBR for mineral oil and the common hydraulic fluids, FKM for higher temperatures and aggressive media.

In practice, two questions usually decide it. If no flaring tool is at hand or tube sizes change often, that argues for the cutting ring, because it needs no forming and the tube is only cut to length and pre-assembled. If the plant is already built around JIC or the line vibrates strongly, that argues for the flare; where maximum vibration and fatigue strength without a metal sealing point count, ORFS is the third option. And where a plant runs several systems side by side, standardize per line or assembly, not per plant: keeping one line entirely on one design avoids adapters, mismatches and extra leak points. Wall thickness plays in too: the cutting ring needs a minimum wall so the cutting edge holds and does not crush the tube, while a tube end can only be flared if it is neither too thin nor too hard.

Before you select or replace a fitting, you have to know the existing thread and sealing seat for certain. Three measurements are enough for the family: the outer diameter with a caliper, the pitch with a pitch gauge and the taper, by comparing the diameter at the frontmost and the rearmost thread. A fourth value finally separates the similar families, the flank angle: 55 degrees carry the Whitworth pipe threads BSP (G and R), 60 degrees the metric ones as well as NPT and the inch UN threads of JIC and ORFS.

The identification only becomes unambiguous in the quartet of diameter, pitch, taper and flank angle: the diameter narrows it down, pitch and taper separate the look-alike families, and the flank angle decides between Whitworth and the 60-degree threads. Note all four before you open a chart. On inch fittings the dash number denotes the tube outer diameter in sixteenths of an inch, with ORFS sitting one dash number lower than JIC at the same thread.

Five thread families dominate hydraulics: BSP with the parallel version G and the tapered R, the metric ISO fine thread, the American NPT, the JIC 37-degree flare and ORFS. Parallel threads (G, M, JIC, ORFS) have the same diameter front and back, tapered ones (R, NPT) narrow toward the tip with a 1:16 taper. But here too: the same thread does not mean compatible. JIC in size -06 and ORFS in -04 both carry the thread 9/16-18 UNF and screw together by hand, yet never seal, because one has a 37-degree flare cone and the other a flat O-ring face. The full measuring protocol with size charts for every family and an interactive tool is in the article identify hydraulic threads; the families with sealing principle, standard and a cross-reference by dash size are set side by side in thread types in hydraulics.

Besides the tube side, every fitting has a port side, the stud end to the manifold or valve, and its sealing seat belongs in the inventory too. The port forms are governed by DIN 3852: form A with an inserted seal ring, form B with a metallic sealing edge and form E with an O-ring at the stud base. The thread is the same on all three; they are told apart by the sealing seat. This form later determines both the tightening torque and the typical leak cause, which is why you should record it right away.

The cutting-ring fitting per DIN 2353, internationally DIN EN ISO 8434-1, is split into three series that differ in wall thickness, thread and rated pressure: LL (very light), L (light) and S (heavy). The series follows the pressure, the size follows the tube outer diameter in millimetres, not the bore. At Parker the three series are called EO Very Light, EO Light and EO Heavy.

Every DIN 2353 fitting carries two different threads: the tube-side union-nut thread at the 24-degree cone and the stud thread that connects to a manifold or valve. These two are not the same; a 12 mm tube in series L, for example, carries the union nut M18x1.5 but the stud M16x1.5. On the same cone the standard also knows two sealing variants, the purely metallic cutting ring and the DKO version with an additional O-ring, which share series, thread and rated pressure.

LL covers small control and measuring lines from 4 to 12 mm at a constant 100 bar. L is the standard in general machine building, covers the widest size range with 6 to 42 mm and carries 160 to 315 bar in the DIN baseline, up to 500 bar in premium steel. S is the high-pressure series from 6 to 38 mm with up to 630 bar in the DIN baseline and up to 800 bar in premium steel; the 630 bar value for the small sizes S 6 to S 14 only applies with a conical counter-thread, otherwise it is 400 bar.

The three series are not interchangeable. Even at the same tube outer diameter they carry different threads: a 12 mm tube has the union nut M18x1.5 in series L, but M20x1.5 in series S. Union nut, cutting ring and stud must therefore always come from the same series. Day to day, series L carries most of the work; LL stays reserved for small secondary lines up to 100 bar, and S takes over as soon as the pressure exceeds the L limit or pressure surges come into play. The full thread and pressure tables per size are in the article DIN 2353: the LL, L and S series.

The cutting ring only seals when it cuts cleanly into the tube, and that comes from the right number of turns, not from feel. DIN 2353 requires a two-stage, turn-controlled assembly per DIN 3859: first the pre-assembly, in which the ring raises a visible collar of material ahead of its cutting edge (the Bundaufwurf), then the final assembly in the fitting body. The prerequisite is a tube cut square and burr-free; a slanted or burred cut prevents the clean raised collar.

From the finger-tight state the union nut turns on by about 1 1/4 to 1 1/2 turns in pre-assembly, depending on the manufacturer; in final assembly, after the clearly felt rise in force, about 1/4 turn for steel and 1/2 for stainless follow, optimized methods only 30 degrees. The inspection criterion is the raised collar: if the encircling collar of material fills the front of the edge and the ring can rotate but no longer move axially, the pre-assembly is correct. The full step-by-step guide with the turn counts per manufacturer and the most common mistakes is in assembling a cutting-ring fitting.

Pre-assembly is done in a hardened pre-assembly stud or directly in the fitting body. The hardened stud protects the reusable body and delivers a reproducible bite; it is replaced after about 50 pre-assemblies. For small sizes and single assembly, the one-stage direct assembly in the body without a separate stud is often enough; for series assembly, large sizes and stainless steel, pre-assembly in the hardened stud stays the safe choice.

Most leaking cutting-ring fittings trace back to an assembly mistake, not to a defective part. Typical are a tube cut slanted or burred, a tube not pushed up to the stop, too few turns with the rise in force ignored, or an over-tightened nut that crushes the ring and constricts the tube. Almost every one of these is avoidable with a clean pre-assembly and the visual check of the raised collar.

When tightening a DIN 2353 fitting you have to separate two force points and two methods cleanly. The stud end, that is the port thread to the manifold or valve, is tightened torque-controlled with a torque wrench; for it there are verified Nm tables by series, thread and sealing form. The tube-side union nut with the cutting ring, by contrast, is assembled not to a torque but turn-controlled. A torque wrench does not replace the turn assembly at the ring; it belongs on the stud.

So why do so many seemingly contradictory tightening-torque tables circulate? Because DIN 2353 only standardizes the geometry, not the torque. The values depend on lubrication, coating (friction-coated nuts about 30 percent lower), material (aluminum bodies at least 30 percent lower), sealing form and connection type, and two further systems on the same 24-degree cone, the metallic profile ring and the DKO soft seal, publish their own values. Never average two tables; take the value that fits the fitted brand. The verified Nm tables for the L and S series by sealing form are in tightening torques for cutting-ring fittings; the series themselves are explained in the DIN 2353 article.

In practice this means: first clarify the connection type and the sealing form of the stud (form A, B or E), and the method follows from that. You tighten the stud end torque-controlled by series, thread and sealing form and keep the thread clean and lightly oiled, because the table values apply to phosphated and oiled. The tube-side union nut you assemble turn-controlled, separated into pre-assembly and final assembly. After the first pressure cycle, check the seat and, if needed, re-set until the force rises again.

If a fitting leaks, depressurize the system first, before the wrench goes on: pump off, system pressure to zero, secured against restart, and relieve accumulators and standing oil columns too. Retightening a line under pressure can destroy the sealing point for good and release a fine oil jet that breaks through the skin.

Retightening only helps while the fitting is not yet tight. If the union nut is already tight and still leaks, the sealing point is damaged, and tightening further presses the cutting ring deeper, pinches the O-ring or cracks the flare cone. So first determine the sealing principle and read the leak pattern accordingly: a leaking cutting ring without a raised collar wants controlled re-assembly, an aged O-ring wants replacing, an angle conflict of R and NPT wants replacing with the correct standard. The cause table by sealing form and the five-step diagnosis are in the article leaking hydraulic fitting; if the assignment is not obvious at once, determine the thread and sealing seat cleanly on the part.

Two causes are behind most cases: wrong tightening and a damaged sealing element. Fittings left too loose never seal properly, while over-tightened ones destroy the ring, flare or O-ring. Look for the cause at the sealing form rather than at tightening alone, and you replace in a targeted way instead of retightening again and again. The diagnosis follows in five steps:

Clear cases for replacement, where further retightening no longer helps, are: