Metric Hydraulic Fitting Identification Guide: Threads, 24° Cones and L/S Series

Metric hydraulic fittings are not identified by thread diameter alone. Two connections can both have metric threads and still be incompatible because their pitch, sealing face, tube outside diameter or fitting series is different. A connection may screw together for several turns yet fail to create the seal it was designed to make.

This practical guide explains how to identify an unknown metric hydraulic fitting before ordering a replacement. It focuses on the information that should appear on a purchase request or drawing: connection type, male or female end, thread size and pitch, sealing geometry, tube outside diameter, light or heavy series, material and fitting shape.

Safety note: Never loosen or inspect a fitting while a hydraulic system is pressurized. Isolate the machine, release stored pressure and follow the equipment manufacturer's safety procedure. Measurements and photographs are screening tools; final compatibility should be checked against the applicable standard or product drawing.

What Does “Metric Hydraulic Fitting” Actually Mean?

“Metric” describes the thread measurement system. It does not, by itself, identify how the connection seals.

A metric thread may be used with:

• a 24-degree cone tube connection;

• a cutting ring assembly;

• an O-ring-assisted 24-degree cone;

• a metric stud end with an elastomeric seal;

• a flat sealing face;

• a swivel nut; or

• a manufacturer-specific connection.

That is why an enquiry such as “M18 fitting” is incomplete. A useful specification would describe the full connection—for example, the measured thread, pitch, sealing form, tube OD and series—then confirm it against a drawing.

Tools Needed for Identification

For a reliable first inspection, prepare:

• a clean vernier or digital caliper;

• metric thread-pitch gauges;

• inch thread-pitch gauges for comparison;

• a good light and clean lint-free cloth;

• a straightedge or cone-seat gauge where available;

• the mating fitting, port or tube if it can be inspected safely; and

• the supplier's catalogue or dimensional drawing.

Do not use a ruler as the only measuring tool. Small differences in diameter or pitch can separate two incompatible thread families.

A Seven-Step Identification Process

Step 1: Record the Complete Fitting

Before measuring, photograph the complete component from more than one angle. Record whether it is a straight adapter, elbow, tee, bulkhead, plug or hose fitting. Note every connection end separately; an adapter can combine two different standards and sizes.

If the fitting carries a legible part number or manufacturer mark, record it before cleaning. A valid part number can be more useful than appearance alone.

Step 2: Determine Male or Female and Parallel or Tapered

A male connection has external threads; a female connection has internal threads. Then inspect whether the thread is parallel or tapered.

Metric tube-fitting threads are commonly parallel. If the thread diameter changes noticeably along its length, do not assume it is a metric 24-degree connection. It may be a tapered pipe thread or another family that requires a different identification path.

Step 3: Inspect the Sealing Surface

Look past the threads and identify the surface that creates the pressure seal. Common clues include:

• an internal 24-degree cone;

• a mating cone with an O-ring;

• a cutting ring and tube nut;

• an O-ring near the base of a male stud thread;

• a flat face or bonded seal; or

• no separate sealing face, which may indicate a tapered-thread system.

The threads usually supply clamping force. The cone, O-ring, cutting ring or washer is what actually seals the connection.

Step 4: Measure the Thread Diameter

Use the caliper across the crests of a clean male thread. For a female thread, direct measurement is less exact, so combine the caliper reading with a pitch gauge and the mating-part geometry.

Record the measured value rather than immediately rounding it to a familiar nominal size. Coating thickness, wear, caliper position and measurement technique can shift the reading slightly.

Write the result in a traceable form, such as:

Male thread outside diameter measured approximately ___ mm

Step 5: Measure the Thread Pitch

Metric pitch is the distance in millimetres from one thread crest to the next. Place a metric pitch-gauge leaf against the clean thread and check that the teeth seat fully without visible gaps.

Do not confuse metric pitch with threads per inch. If the fit is uncertain, compare both metric and inch gauges. Visually similar BSP, JIC/SAE and metric connections can produce expensive mistakes when the pitch is guessed.

When the size is confirmed, a metric thread is written in a form such as Mxx × pitch; both parts are required.

Step 6: Measure the Tube Outside Diameter

For a metric 24-degree tube connection, the tube outside diameter is a primary identification dimension. Measure the tube itself where possible, not the nut or the fitting body.

Thread size alone may not distinguish the intended tube connection and series. Record the tube OD together with the thread measurement and sealing geometry.

Step 7: Confirm the Series and Drawing

Metric 24-degree tube fittings are commonly supplied in light and heavy series, often abbreviated L and S. The series is not a subjective description of product weight. It identifies a standardized dimensional family with its own relationship among tube OD, thread and connection geometry.

Compare all measured data with the supplier's drawing or the relevant standard. If any one feature conflicts, stop and recheck rather than selecting the closest-looking part.

How a Metric 24-Degree Cone Connection Works

The 24-degree connection family is widely used for hydraulic tube and hose systems. Depending on the design, the assembly can use a cutting ring, a formed tube end or an O-ring-assisted cone.

In a cutting-ring tube connection, the nut drives the cutting ring into the fitting cone during assembly. The ring grips the tube and contributes to the seal. Correct tube preparation, assembly procedure and inspection are essential; replacing only one component without confirming compatibility is poor practice.

In an O-ring-assisted design, the elastomeric seal is part of the intended sealing geometry. An O-ring does not make an otherwise incompatible cone or thread interchangeable.

Light (L) vs Heavy (S) Series

L and S series can share the general appearance of a metric 24-degree fitting, but they are separate dimensional systems. The correct series must be established from the combined tube OD, thread and dimensional drawing.

Avoid these assumptions:

• a larger hexagon automatically means S series;

• the same-looking nut means the complete assembly is interchangeable;

• matching thread diameter proves the cone and tube size match; or

• a component can be moved between L and S systems because it can be tightened.

If an old fitting is damaged or mixed with unidentified parts, compare it with a verified assembly rather than relying on one loose nut.

Metric Fitting Shapes and End Combinations

After identifying the connection, specify the body configuration:

• Straight adapter: joins two ends on the same axis.

• Elbow: changes direction, commonly by 45 or 90 degrees.

• Tee: creates a branch connection.

• Bulkhead: passes through and mounts to a panel.

• Adjustable stud fitting: allows orientation before final tightening.

• Swivel connection: permits assembly without rotating the hose or tube.

• Plug or cap: closes a specified male or female end.

Common Identification Mistakes

Calling Every Millimetre Measurement “Metric”

A caliper displays millimetres regardless of the thread system. Measuring an inch-based thread in millimetres does not turn it into a metric thread. Confirm the pitch and profile.

Identifying from Thread Diameter Only

Diameter is only one clue. Pitch, sealing face, tube OD and series must agree. A connection that starts by hand is not proof of compatibility.

Confusing BSPP with a Metric Parallel Thread

Both can appear parallel and may have similar measured diameters. Their thread form, pitch and associated sealing arrangements differ. Use the correct pitch gauges and inspect the seal.

Confusing JIC with a Metric Cone

JIC uses an inch-based straight thread and a 37-degree flare sealing system. A metric 24-degree connection uses different geometry. Similar overall shape does not make them interchangeable.

Ignoring the Mating Half

The male fitting alone may not reveal the full assembly. Inspect the female seat, port, tube, nut, ring and O-ring arrangement whenever possible.

Reusing Damaged Sealing Components

Scratched cones, deformed cutting rings, flattened O-rings and damaged threads can cause leakage even when the nominal specification is correct. Identification and condition assessment are separate steps.

Information to Send When Requesting a Replacement

A complete enquiry should include:

For mixed orders, use one line per unique configuration. Avoid descriptions such as “same as photo” unless a verified sample and critical dimensions are also supplied.

A Practical Photo Checklist

For remote identification, send:

1. one photo of the complete fitting beside a scale;

2. one straight-on photo of each sealing face;

3. one photo showing the caliper across the male thread;

4. one photo showing the pitch gauge seated in the thread;

5. one photo of the tube, nut and ring arrangement; and

6. any legible part number, marking or original drawing.

Use sharp, well-lit images and keep confidential customer information outside the frame or redact it before sending.

Frequently Asked Questions

Are all metric hydraulic fittings 24-degree cone fittings?

No. Metric threads are used with several sealing systems. Always identify the sealing face and mating component.

Can I identify L or S series from the thread alone?

Not reliably. Confirm the relationship among thread size, pitch, tube outside diameter and the applicable dimensional drawing.

Is a metric fitting compatible if it screws into the port?

No. Partial thread engagement does not prove correct pitch, thread profile or sealing geometry. Forcing an incorrect connection can damage both components.

Should thread sealant be applied to a 24-degree cone connection?

The 24-degree sealing interface is designed around its specified cone, ring or O-ring arrangement. Do not use sealant as a substitute for correct components or an undamaged sealing face. Follow the fitting manufacturer's assembly instructions.

What is the fastest way to obtain an accurate quotation?

Provide the original part number or drawing when available. Otherwise send the complete measurement and photo set, tube or hose data, material, quantity and application requirements.

Source Metric Hydraulic Fittings with Clear Specifications

Ruihua Hardware supplies metric hydraulic adapters, hose fittings and related connection components in straight, elbow, tee, bulkhead and mixed-end configurations. Review our hydraulic adapters, hydraulic hose fittings and metric hydraulic hose fitting guide.

For an overview of other connection families, read Hydraulic Fitting Types Explained. If you need help matching an existing component, contact Ruihua Hardware with the thread measurements, sealing-face photographs, tube or hose data and required quantity. Our team can review the information against available drawings before quotation.