How are threaded fittings marked for material and thread identification?

Threaded fittings look like simple plumbing hardware, but the stamps and codes pressed into their bodies are the only way to confirm that the part you are about to install will actually meet the pressure, temperature, and corrosion demands of your system. Reading those markings correctly is a routine skill for any piping engineer, procurement officer, or maintenance technician. This guide walks through every category of marking that appears on a modern threaded fittings catalog, explains what the codes mean, and shows how to match them to the real-world service conditions on your project.

Why Markings Matter More Than Datasheets

Paperwork can be lost, photocopied, or misfiled; a casting stamp cannot. For that reason, the MSS SP-25 standard, the ASME B16 product family, and most national standards (EN, JIS, GB/T) all require that the fitting itself carry the essential information on its body. If a fitting does not show the standard, the pressure class, the material identifier, and the manufacturer's mark, treat it as a red flag and do not install it.

The general order of markings on a forged or machined threaded fitting is roughly:

Manufacturer's name or symbol
Material identification (for example 304, 316, A105, B165)
Standard the fitting was produced to (ASME B16.11, EN 10241, BS 21, JIS B2301, etc.)
Pressure class or thread designation (CL2000, CL3000, CL6000, or PN rating)
Size (nominal diameter, e.g. 1/2, 2, DN50)
Heat number or batch code for traceability

Material Identification: What the Stamps Actually Mean

Material codes are the most important stamps because they decide whether the fitting survives the medium it carries. The two dominant systems in use today are ASTM/ASME (United States) and EN (Europe), with JIS marks common on parts from East Asia.

Carbon and Alloy Steel Forgings

ASTM A105 is the workhorse specification for carbon steel forged threaded fittings up to 540 °C. You will usually see it stamped simply as "A105". Higher-temperature service uses A182 (forged or rolled alloy), with grades such as F11, F22, F91 marked accordingly. For low-temperature service down to −46 °C, the marking becomes "A350 LF2". A code like "F316" means the part is made of forged 316 stainless steel under A182.

Stainless Steel

On stainless steel fittings the material is normally written as the Unified Numbering System (UNS) code or the AISI/SAE grade: 304, 304L, 316, 316L, 321, 347, or duplex 2205. The "L" indicates low carbon (≤ 0.03 %) and is essential for welded systems where sensitization must be avoided. For an austenitic stainless used in marine heat exchangers you will often see the UNS equivalent, e.g. "S31600" for 316 or "S31803" for 2205 duplex. The same UNS family shows up on heat exchanger tube hangers and pipe flanges, so reading one set of markings helps you read them all.

Copper, Copper-Nickel and Nickel Alloys

Marine and desalination systems frequently use copper-nickel fittings. Look for the UNS codes C70600 (90/10 Cu-Ni) and C71500 (70/30 Cu-Ni), or the standard reference such as EEMUA 234 or BS 2871. For higher-temperature or more aggressive media the part may be marked B165 (Monel 400) or B407 (Incoloy 800), referencing the ASTM specification that controls chemistry and mechanical properties.

Thread Identification: NPT, BSP and Metric

Material tells you what the fitting is made of; thread designation tells you what it will connect to. Mixing the two is the single most common cause of leaks in cross-border projects.

NPT — National Pipe Taper (ANSI/ASME B1.20.1)

NPT is the dominant thread in North America. It is a 60° tapered thread that seals by thread deformation, so it almost always requires sealant. On a fitting you will see one of the following:

"NPT" — standard dryseal taper, e.g. "1/2 NPT"
"NPTF" — "dryseal" thread that can seal without sealant under certain conditions
"MIP" / "FIP" — male/female iron pipe references in older documentation

BSP — British Standard Pipe (ISO 228, ISO 7, BS 21)

Most of Europe, the Middle East, and Asia use BSP. There are two variants and they are not interchangeable:

BSPT (R / Rc) — 55° tapered thread, per ISO 7-1. Sealing is on the thread, usually with PTFE tape or a paste sealant.
BSPP (G) — 55° parallel thread, per ISO 228-1. The thread itself does not seal; a bonded seal, O-ring, or metal-to-metal washer is required on the face.

A fitting will be stamped "G1/2" or "R1/2" to show which it is. Because both have a 55° flank angle they can be confused with each other on the shop floor, but a parallel male will bottom out in a tapered female before it seals, and a tapered male will crush a bonded seal in a parallel female. Always read the letter as carefully as the number.

Metric Parallel Threads

ISO metric threads use the letter "M", followed by the nominal diameter and pitch in millimetres, for example "M20 × 1.5". Metric parallel threads are common on European instrumentation and hydraulic equipment. They never seal on the thread alone, so look for a 24° cone, an O-ring face seal (ORFS), or a DIN 3852 port when you see an M-marked fitting.

Pressure Class: Reading CL and PN Stamps

The class stamp tells you how much pressure the fitting can hold at a reference temperature. The ASME B16.11 forged fitting classes are:

Class 2000 — light-duty, equivalent to about 140 bar at room temperature
Class 3000 — standard for most process piping (≈ 210 bar)
Class 6000 — high-pressure service (≈ 420 bar)
Class 9000 — sour-service and high-pressure hydraulic systems

European PN ratings use the same idea but in bar at ambient temperature: PN 10, PN 16, PN 25, PN 40, and so on. The marking is normally placed next to the size, e.g. "DN 50 PN 40". PN and Class are not interchangeable numbers; a Class 3000 forged fitting is rated very differently from a PN 300 bar fitting, so the marking must be read in full.

How to Read a Real Marking, Step by Step

Suppose you pick up a 90° elbow from a recent delivery and see the following cast into the body:

EZ • 316 • B16.11 • CL3000 • 2 • H2024-1187

Translate it as follows:

EZ — manufacturer's mark (the supplier's code).
316 — 316 stainless steel body, suitable for corrosive service.
B16.11 — produced to ASME B16.11, the standard for forged steel threaded fittings.
CL3000 — pressure class 3000, ≈ 210 bar at room temperature.
2 — nominal size 2 inch (DN 50).
H2024-1187 — heat number, traceable back to the original steel melt and the MTC.

If any of these six items is missing, the chain of custody is broken. Cross-check the heat number against the mill test certificate before you bolt the fitting into a hydrocarbon, chemical, or high-temperature line.

Practical Tips for the Field

Always verify that the thread code on the fitting matches the receiving port. A 1/2 NPT male will not seat in a 1/2 BSPT female even though the diameter is identical.
Keep a small pocket chart of the four major thread systems (NPT, BSPT, BSPP, metric) at the toolbox. A 30-second check prevents a multi-day rework.
For stainless steel fittings, do not rely on a magnet test alone. Use the stamped UNS or grade and a portable XRF analyser for critical service.
On piping that includes pipe flanges as well as threaded fittings, check that the flange class and the fitting class are consistent; a CL150 flange feeding a CL3000 threaded elbow is a design error.
For systems that also carry heat exchanger tube bundles, the fitting material should match the tube material (for example, 316 tubes with 316 headers and 316 threaded plugs) to avoid galvanic corrosion at the tube-sheet joint.

FAQ

Can a fitting be NPT and BSP at the same time?

No. NPT and BSP use different thread angles (60° versus 55°) and slightly different pitches. A hybrid fitting must be marked for one system only, and adapters must be used to cross between them.

What if the material stamp is missing but the certificate is in the file?

Treat the fitting as non-conforming. Standards require the mark to be on the part, not just on the paperwork. Send the part back for re-stamping or ｒｅｐｌａｃｅ it.

Are stainless markings universal across ASTM, EN and JIS?

The chemical designations are very close (304 ≈ 1.4301 ≈ SUS304; 316 ≈ 1.4401 ≈ SUS316), but the allowable composition ranges are not identical. For critical service, verify the full chemistry on the MTC, not just the grade number on the body.

Final Thoughts

A threaded fitting is essentially a small data plate made of metal. Once you can read manufacturer, material, standard, class, thread and size in that order, you can verify every component on the jobsite without opening a single datasheet. Pair that habit with a clear thread reference card and consistent inspection on delivery, and you will avoid the most common fitting failures: wrong material, wrong pressure rating, and wrong thread family. EZ Steel Industrial supplies a full range of threaded fittings, pipe flanges, stainless steel piping, and heat exchanger tube products, all marked to the relevant international standard, so that the identification skills above translate directly into trouble-free installations.