Overview
The compression straight tube fitting is a complete three-piece coupling – body, cutting ring, and union nut – joining two tubes of the same outside diameter, per DIN 2353 / ISO 8434-1 with a 24° internal cone. Tightening the nut drives the ring into the cone, biting the tube wall to a metal-to-metal seal – no welding or flaring.
This page covers the complete assembly; the ring and nut are components, not standalone items – for replacements, confirm at order.
Specifications
LL, L, and S are dimensional series: wall thickness and nut thread define the series; pressure is the result per size.
| Series | Tube OD | Material | Working pressure by size |
|---|---|---|---|
| LL (extra light) | 4-8 mm | Carbon steel / stainless | 100 bar |
| L (light) | 6-42 mm | Carbon steel / stainless | 250 bar (6-15 mm); 160 bar (18-22 mm); 100 bar (28-42 mm) |
| S (heavy) | 6-38 mm | Carbon steel | 630 bar (6-12 mm); 400 bar (16-25 mm); 250 bar (30-38 mm) |
| S (heavy) | 6-25 mm | Stainless 1.4571 / 1.4404 | 400 bar (6-12 mm); 250 bar (16-25 mm) |
| Tube OD | Nut thread | Hex (mm) | Length (approx.) | L series pressure | S series pressure (carbon steel) |
|---|---|---|---|---|---|
| 6 mm | M12x1.5 | 14 | 33 mm | 250 bar | 630 bar |
| 8 mm | M14x1.5 | 17 | 36 mm | 250 bar | 630 bar |
| 10 mm | M16x1.5 | 19 | 37 mm | 250 bar | 630 bar |
| 12 mm | M18x1.5 | 22 | 43 mm | 250 bar | 630 bar |
| 15 mm | M22x1.5 | 27 | 48 mm | 250 bar | 400 bar |
| 18 mm | M26x1.5 | 32 | 48 mm | 160 bar | 400 bar |
| 22 mm | M30x2 | 36 | 52 mm | 160 bar | 400 bar |
| 28 mm | M36x2 | 41 | 58 mm | 100 bar | – |
| 35 mm | M45x2 | 50 | 72 mm | 100 bar | 250 bar |
| 42 mm | M52x2 | 60 | 78 mm | 100 bar | – |
S series is not declared at 28 mm and 42 mm – confirm at order.
| Body material | Temperature range | Typical use |
|---|---|---|
| Zinc-plated carbon steel | -20°C to +120°C | General hydraulics, industrial machinery, steel tube |
| Stainless steel 1.4571 (316Ti) | -60°C to +400°C | Chemical, marine, high-temperature lines; S series max 400 bar |
| Stainless steel 1.4404 (316L) | -60°C to +400°C | Corrosive or washdown environments; confirm seal compatibility at order |
| Brass C36000 | -54°C to +121°C | Low-pressure lines at 10-28 bar; copper, brass, or aluminum tube |
Seal temperature limits: NBR -35°C to +100°C; FKM -25°C to +200°C; PTFE -200°C to +260°C.
| Tube OD (imperial brass) | Thread | Hex | Working pressure |
|---|---|---|---|
| 1/8″ (3.18 mm) | 5/16-24 | 7/16″ | 400 PSI (28 bar) |
| 3/16″ (4.76 mm) | 3/8-24 | 1/2″ | 400 PSI (28 bar) |
| 1/4″ (6.35 mm) | 7/16-24 | 9/16″ | 300 PSI (21 bar) |
| 5/16″ (7.94 mm) | 1/2-24 | 5/8″ | 300 PSI (21 bar) |
| 3/8″ (9.53 mm) | 9/16-24 | 11/16″ | 200 PSI (14 bar) |
| 1/2″ (12.7 mm) | 11/16-20 | 7/8″ | 200 PSI (14 bar) |
| 5/8″ (15.88 mm) | 13/16-18 | 1″ | 150 PSI (10 bar) |
| 3/4″ (19.05 mm) | 1-18 | 1-1/8″ | 100 PSI (7 bar) |
| Tube OD | L series min. wall | S series min. wall |
|---|---|---|
| 6 mm | 1.0 mm | 1.5 mm |
| 8 mm | 1.0 mm | 1.5 mm |
| 10 mm | 1.0 mm | 1.5 mm |
| 12 mm | 1.0 mm | 2.0 mm |
| 15 mm | 1.5 mm | 2.5 mm |
| 18 mm | 1.5 mm | 3.0 mm |
| Tube OD | L series torque | S series torque |
|---|---|---|
| 6 mm | 15 Nm | 20 Nm |
| 8 mm | 20 Nm | 25 Nm |
| 10 mm | 25 Nm | 35 Nm |
| 12 mm | 30 Nm | 45 Nm |
| 15 mm | 35 Nm | 55 Nm |
| 18 mm | 40 Nm | 70 Nm |
| 22 mm | 50 Nm | 90 Nm |
Applications
- Hydraulic power units and mobile machinery (L and S ranges).
- Process and industrial piping: dosing, sampling, distribution, instrument lines.
- Marine and offshore: deck hydraulics, ballast, and fuel transfer lines in stainless.
- Corrosive or washdown environments: stainless bodies; confirm seal compatibility at order.
- Low-pressure pneumatics, cooling, and plumbing: LL metric or imperial brass.
Series and Material Selection
Pick the series from the tube, not a pressure label: LL for 4-8 mm light tubing, L for 6-42 mm general hydraulics, S for 6-38 mm heavy service. Larger OD in a series means lower pressure – see the tables and our tubing and fittings selection guide.
Match fitting material to tube material per the table above. The seal, not the body, sets the practical temperature ceiling; phosphate ester fluids require FKM seals – confirm at order.
Above +100°C, de-rate per ISO 8434-1 – near +200°C, roughly 80% of nominal. Verify the weakest component – body, tube, seal, or port. Request a quote with tube OD, wall, medium, and temperature.
Installation
Tube preparation decides the seal: cut square with a tube cutter (not a hacksaw), deburr both edges, clean, verify OD with calipers, meet the minimum wall.
- Slide nut, then ring (taper toward the body), onto the tube.
- Push the tube firmly against the internal stop.
- Hand-tighten until resistance.
- Hold the body; turn the nut with a second wrench.
- L series: about 1.5 turns past finger-tight; S series: per torque table.
- Verify the ring’s front edge has visibly bitten the tube.
The cutting ring is single-use: after disassembly, fit a new ring and cut off the bitten tube section; body and nut are reusable.
Steel and stainless rings will not seal on copper or aluminum – use brass fittings for soft tube.
Maintenance
Inspect joints regularly for weeping, corrosion, and loose nuts; interval depends on conditions, not a calendar. Keep spare cutting rings and union nuts in stock. Material certificates on request – confirm at order. For fitting-type background, see our grease fitting types guide.
FAQ
What is a DIN 2353 compression straight tube fitting?
A complete three-piece coupling – body, cutting ring, union nut – per DIN 2353 / ISO 8434-1; the nut drives the ring into the 24° cone, biting the tube into a metal-to-metal seal.
What is the difference between LL, L, and S series?
Dimensional series, not pressure classes: LL is 4-8 mm at 100 bar, L is 6-42 mm at 100-250 bar, S is 6-38 mm at 250-630 bar (carbon steel), each with its own threads and minimum walls.
Can I reuse the cutting ring after disassembly?
No – single-use, permanently formed to one tube; reassemble with a new ring after cutting off the bitten section. Body and nut are reusable.
How do I choose between carbon steel, stainless, and brass?
Match the tube: zinc-plated carbon steel for steel tube (-20°C to +120°C), stainless 1.4571 / 1.4404 for corrosive or hot duty (-60°C to +400°C), brass C36000 for soft tube (10-28 bar).
What are the maximum pressure and temperature limits?
630 bar for carbon steel S (6-12 mm), 400 bar for stainless S; seals cap temperature (NBR +100°C, FKM +200°C, PTFE +260°C); de-rate above +100°C per ISO 8434-1.
How do I prepare the tube before assembly?
Cut square with a tube cutter, deburr edges, clean, verify OD with calipers, meet the minimum wall, and seat it on the internal stop.
Can I use copper or aluminum tube with a steel DIN 2353 fitting?
No – soft tube lets the steel ring over-cut and leak; use brass fittings for copper or aluminum.
What is the correct tightening procedure?
Seat the tube on the stop, hand-tighten, then wrench-tighten holding the body: about 1.5 turns past finger-tight for L, torque table (20-90 Nm) for S.
How do I tell whether my fitting is L or S series?
Measure tube OD and nut thread; check the nut marking. At the same OD, S uses a larger thread – at 12 mm the minimum wall is 1.0 mm (L) versus 2.0 mm (S).
What spare parts should I keep in stock?
Cutting rings and union nuts – every disassembly consumes a ring. For a matched set, contact our team for a quote from IsoHiTech Lubrication Systems, founded in 2023, engineering since 2006, or email inquiry@isohitech.com.
