TL;DR: S and L series are the two most common types of DIN 2353 metric tube fitting, and the letter tells you the pressure class, not the size. L series (light) is the medium-pressure option, rated up to around 500 bar, with thinner tube walls and a smaller nut across the same tube diameters. S series (heavy) has thicker walls and a larger body, and is rated up to around 800 bar for high-pressure and high-vibration duty. Both use the same 24 degree cone and a compression (bite) ring to seal on the tube, so an L series and an S series fitting for the same tube outside diameter will not interchange because the nut, cutting ring and body dimensions differ. Separately, the connection thread can be metric or BSP: metric threads have a 60 degree form and are sized in millimetres (for example M18x1.5), while BSP threads have a 55 degree form and are sized in inches (for example 1/2 inch BSP), and the two are not compatible. Hydraulic & Engineering Services (HESL) stocks S and L series tube fittings, adaptors, and hydraulic tube ex-stock at its Strood trade counter and can identify an unmarked fitting for you. Check availability or ask us to identify your fitting.
What are S and L series hydraulic fittings?
S and L series hydraulic fittings are metric tube couplings built to the DIN 2353 standard. They join rigid hydraulic tube to ports, hoses, manifolds and other tube runs, and they seal by compressing a hardened ring onto the tube behind a 24 degree cone inside the fitting body.
The letters S and L are pressure and dimension series, not tube sizes. For any given tube outside diameter, the L series version is lighter with a thinner wall and a smaller nut, and the S series version is heavier with a thicker wall and a larger nut. So a 12L fitting and a 12S fitting both suit 12 mm tube, but they are physically different parts rated for different pressures. This catches a lot of people out at the parts counter, because they ask for a “12 mm fitting” without saying whether the system is an L or an S build.
There is also a third, less common band, the LL series (extra light), used for very low-pressure instrumentation and small-bore work. In day-to-day hydraulics the choice is almost always between L and S.
What does DIN 2353 mean?
DIN 2353 is the German and widely adopted international standard that defines compression tube fittings with a 24 degree cone. It fixes the geometry of the cone, the cutting or progressive ring, the nut and the body so that fittings and tube from different manufacturers work together predictably.
The important thing to understand is that DIN 2353 describes the tube end of the connection: how the fitting grips and seals onto the rigid tube. The other end of the fitting, the part that screws into a port or another component, can be threaded in several different standards, which is where metric and BSP come in later in this guide. The equivalent international standard is ISO 8434-1, so a supplier quoting “ISO 8434-1” and one quoting “DIN 2353” are describing the same 24 degree cone system.
Because the standard is shared, an S series body from one maker will mate with an S series tube run cut and rung by another, provided the series, tube diameter and materials match. That interchangeability is exactly why these fittings dominate European mobile and industrial hydraulics.
S series vs L series: what is the difference?
The difference is pressure rating and physical size, driven by wall thickness. S series carries more pressure because it is a heavier build. The table below sets out how the two series compare.
| Feature | L series (light) | S series (heavy) |
|---|---|---|
| Duty | Medium pressure | High pressure and high vibration |
| Typical max pressure | Up to around 500 bar | Up to around 800 bar |
| Tube wall and body | Thinner wall, smaller nut and body | Thicker wall, larger nut and body |
| Typical tube sizes | 6L to 42L | 6S to 38S |
| Common uses | General industrial circuits, machine tools, lighter mobile plant | Heavy plant, mining, shipbuilding, presses, breakers and demolition attachments |
| Interchangeable with the other series | No | No |
Pressure ratings vary by tube diameter and by manufacturer, and the figures above are the accepted series maximums rather than a rating for every size. Smaller tube diameters within a series carry higher pressure than the larger diameters, so always check the specific size on the datasheet before you specify. As a working rule, if the circuit is a heavy-duty or shock-loaded application such as a grab, breaker or press, it is almost certainly an S series build.
What is the LL series, and when is it used?
The LL series (extra light) is the lightest DIN 2353 band, intended for very low-pressure, small-bore work such as instrumentation lines, pilot lines and gauge connections. It uses thinner tube again than the L series. Most hydraulic power circuits do not use it, but you will occasionally meet it on control and sensing lines, so it is worth recognising the designation when you strip a machine down.
Metric vs BSP: how do you tell hydraulic threads apart?
The DIN 2353 cone deals with the tube end. The port or coupling end carries a thread, and in UK hydraulics that thread is usually either metric or BSP. Getting this wrong is the single most common cause of a leaking or cross-threaded connection. The two are not interchangeable, because the thread forms are cut to different angles.
| Feature | Metric thread | BSP thread |
|---|---|---|
| Thread angle | 60 degrees | 55 degrees |
| Sizing | Millimetres, for example M18x1.5 | Inches, for example 1/2 inch BSP |
| Parallel or tapered | Usually parallel (metric parallel) | Parallel (BSPP) or tapered (BSPT) |
| How it seals | Soft seal or cone, not the thread | Soft seal or bonded washer (BSPP), or thread interference (BSPT) |
| Common on | European machinery, DIN 2353 systems | UK plant, ports, adaptors and legacy equipment |
To identify a thread reliably you need two measurements: the outside diameter of a male thread (or the inside diameter of a female one), and the pitch or threads per inch. A thread pitch gauge and a caliper settle it in seconds. A quoted “M18x1.5” is a metric thread of 18 mm diameter and 1.5 mm pitch. A “1/2 inch BSP” is a British Standard Pipe thread sized in the old imperial pipe convention, which is why the physical diameter never matches the stated size. If you are unsure, bring the fitting to the counter and we will gauge it.
How do you identify which fitting you already have?
If a machine is in front of you and the paperwork is long gone, work through it in order:
- Measure the tube outside diameter in millimetres. That gives you the size number, for example 12 or 16.
- Compare the nut and body size against a known L and S sample. The heavier, larger nut is S series.
- Gauge the port thread for diameter and pitch to decide metric or BSP.
- Check the seal type at the port, whether it is a cone, an O-ring face or a bonded washer.
- Note the material, steel for standard hydraulics or stainless for marine, water and corrosive duty.
Getting all five right first time avoids the classic mismatch where a correctly sized tube fitting simply will not seal because the series or the thread is wrong. If you are also sizing the valves those lines feed, our guide to CETOP valve sizes and flow ratings explains how the interface standard works on the valve side.
Can you mix S and L series, or metric and BSP?
No. You cannot mix an S series nut and ring with an L series body, because the dimensions of the cone seat, the ring and the nut thread all differ between the series. You also cannot run a metric male into a BSP female, or the reverse, because the 60 degree and 55 degree thread forms will not match and will either leak or damage the threads.
Where you genuinely need to move between standards, for example connecting a metric machine to a BSP port, the correct solution is a proper adaptor rated for the pressure, not force-fitting mismatched parts. HESL carries a wide range of adaptors alongside the S and L series range, so a mixed-standard machine can be plumbed correctly rather than bodged.
How do you choose the right hydraulic fitting?
Choosing correctly comes down to four decisions taken in this order: the tube outside diameter sets the size, the working pressure sets the series (L or S), the port sets the thread standard (metric or BSP) and seal type, and the environment sets the material (steel or stainless). Get those four right and the fitting will seal first time and stay sealed.
For anyone rebuilding a circuit, it pays to standardise. Machines that have been patched over the years often carry a mix of L and S fittings and both thread standards, which slows every future repair and multiplies the spares you have to hold. Rationalising to a consistent series and thread where the pressures allow is one of the simplest reliability improvements you can make. If you are replacing perished hoses and fittings during a larger overhaul, our walkthrough of an orange peel grab refurbishment shows where fitting choice fits into a full strip and rebuild.
Where to buy S and L series fittings and hydraulic tube
Hydraulic & Engineering Services stocks S and L series DIN 2353 tube fittings, adaptors and the associated hydraulic tube ex-stock, and can cut, ring and supply made-up tube assemblies from its bar and tube division. The components and trade counter at Strood carries fittings alongside directional control valves, pumps, motors and filtration, so a full circuit can be specified from one source. Bring in an unmarked fitting and the counter team will identify the series, size and thread for you before you buy.
Check stock or ask us to identify your fitting.
Frequently asked questions
What does the L and S mean in hydraulic fittings? L means the light (medium-pressure) series and S means the heavy (high-pressure) series of DIN 2353 metric tube fittings. The letter refers to the pressure class and physical build, not the tube diameter, so a 12L and a 12S both suit 12 mm tube but are different, non-interchangeable parts.
What pressure can S and L series fittings take? L series is generally rated up to around 500 bar and S series up to around 800 bar, but the exact figure depends on the tube diameter and the manufacturer. Smaller diameters within a series carry higher pressures than larger ones, so check the datasheet for the specific size.
Are DIN 2353 and ISO 8434-1 the same? Yes, in practice. Both describe the 24 degree cone compression tube fitting system, so an S or L series fitting is commonly labelled to either standard.
How do I know if a fitting is metric or BSP? Measure the thread diameter and the pitch, then check the thread angle. Metric threads have a 60 degree form and are sized in millimetres such as M18x1.5. BSP threads have a 55 degree form and are sized in inches such as 1/2 inch BSP. The two are not interchangeable.
Can I use a BSP fitting in a metric port? No. The thread forms differ (55 degrees versus 60 degrees), so forcing them together leaks and damages the threads. Use a correctly rated adaptor to move between the two standards.