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Metric 24° Cone Fittings (L and S Series)

Metric 24° cone fittings join metric tube and hose in L (light) and S (heavy) series, using cutting rings, O-ring cones or welded nipples. This sheet explains how they work, how the series are named and what to check before matching a part.

Ref HY-0018ISO 8434-1

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

The 24° cone fitting system is defined in ISO 8434-1 and comes from the older DIN 2353 standard. The body has a 24° internal cone. A cutting ring (bite ring) slides on the tube; when the nut is tightened, the ring bites into the tube and seals against the cone. Later designs replace the ring with a soft seal or a two-edge ring, and hose ends use a 24° male cone with an O-ring (often called DKO). Fittings come in three series: LL (extra light), L (light) and S (heavy). Size is named by the tube outside diameter in millimeters plus the series, such as 12L or 16S.

How to identify

Measure the tube OD in millimeters. Metric tube does not use inch sizes.
Measure the nut thread with a metric pitch gauge. Threads are metric fine, such as M18 x 1.5.
Look inside the body for a 24° cone. Check with a seat angle gauge.
Note the series. L and S series of the same tube size often have different threads, and some threads are shared between different tube sizes in L and S.
Look for an O-ring on hose ends to tell DKO cones from plain cones.

Thread and tube combinations for each series are on the metric L and S series fitting chart.

Parts and terms

Term	What it is	Notes
Body	Fitting with 24° internal cone	Stamped with series and size on many brands
Nut	Threads onto the body and drives the ring	Matched to series and size
Cutting ring	Bites into the tube	Single use once set
Soft seal ring	Ring with an elastomer seal	Used where leak-free design is required
DKO hose end	24° male cone with O-ring and swivel nut	Common on hose assemblies
Weld nipple	Butt-weld or braze end with O-ring	Replaces the cutting ring on welded tube
L series	Light series	Lower pressure range of the two
S series	Heavy series	Thicker parts; different threads in most sizes

Key points

Pre-set cutting rings with the right tool or a hardened pre-set body, then final assemble per the maker.
Do not mix ring and nut brands unless the maker approves it. Ring geometry differs.
Use the correct tube wall and material for the ring. Very soft or thin tubes need inserts.
Pressure ratings by size and series come from ISO 8434-1 and the maker's catalog.
Assembly torque or turns come from the maker. Do not guess.

Common mistakes

Matching by thread alone and picking the wrong series.
Reusing a cutting ring on a new tube.
Ordering an inch tube size for a metric fitting.
Mixing brands of rings and bodies.

Governing standards: ISO 8434-1 (24° cone connectors); DIN 2353 (former German standard for compression fittings). See the metric L and S series fitting chart.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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