DIN 2353: understanding the LL, L and S series

DIN 2353 describes the reusable tube connection over a 24-degree internal cone. When the union nut is tightened, a cutting ring bites into the tube outer diameter and seals metallically on the cone; in the DKO variant an O-ring adds to the seal. The internationally valid version is DIN EN ISO 8434-1, which largely adopted the older DIN 2353. In the trade both designations mean the same system.

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. The sizing basis is always the tube outer diameter in millimetres, not the bore. At Parker the three series are called EO Very Light, EO Light and EO Heavy, which correspond to LL, L and S. Source: DIN EN ISO 8434-1; Parker manufacturer data.

On the same 24-degree cone the standard knows two sealing variants: the cutting-ring version, in which a metallic ring bites into the tube on tightening and seals purely metallically, and the DKO version with an additional O-ring in the seal cone. Both share series, thread and rated pressure. The DKO variant seals elastomerically and metallically at once and is less sensitive to settling and loosening. Source: DIN 3865 (DKO); HANSA-FLEX manufacturer data.

Tube outer diameters from 4 to 12 mm at a constant 100 bar rated pressure: that is the territory of the very light series LL, the series for small control and measuring lines. LL has the smallest threads and the thinnest walls and belongs where space and weight matter and the pressure stays low.

With tube outer diameters from 6 to 42 mm, the light series L covers the widest size span of the three and is the standard in general machine building. The rated pressure drops as the tube diameter rises: from 315 bar in the DIN baseline version at the small sizes to 160 bar at 22 mm and above. For most hydraulic lines in the medium pressure range, L is the right choice.

Up to 630 bar in the DIN baseline and up to 800 bar in premium steel: in the small sizes the heavy series S carries the highest rated pressures of the three, at tube outer diameters from 6 to 38 mm. The footnote on the small sizes matters: the 630 bar value for S 6 to S 14 only applies with a conical counter-thread; with a cylindrically bored port it is 400 bar. S belongs where high pressures, pressure surges or thick tube walls occur.

The series follows the pressure, the size follows the tube outer diameter. Whoever knows the operating pressure and the tube diameter picks the series in one step.

In practice, series L covers the largest share of applications. Reach for LL on small secondary lines, and for S as soon as the pressure exceeds the L limit or the line is exposed to pressure surges. The matching tightening values per series are in Tightening torques for cutting-ring fittings, the step-by-step assembly in Assembling a cutting-ring fitting correctly.