How to Specify Aluminium Fittings: Alloy Grades, Tempers and Thread Standards

A specification that says "aluminium fitting, 1/4 inch NPT" leaves enormous room for the manufacturer to make decisions that profoundly affect what you receive. The alloy could be A380 die cast or 6061-T6 wrought. The surface could be bare, anodised, or powder-coated. The threads could be NPT or BSPP — both called "1/4 inch." Any of these choices changes the performance of the fitting significantly.

Here is the complete specification framework to ensure you get exactly what you need.

Step 1: Specify the Alloy Grade

Aluminium is not one material — it is a family of alloys with significantly different properties. The most important choice for fittings:

For most industrial and pneumatic fittings: specify 6061-T6 or 6082-T6. For aerospace or very high-strength applications: 7075-T6. For complex-geometry die-cast bodies: A380 with post-cast machining of critical surfaces.

Step 2: Specify the Temper

Aluminium alloy temper is critical — the same alloy in different temper conditions has dramatically different strength properties.

T6 temper: Solution heat-treated and artificially aged. The standard for maximum strength in 6061 and 6082. Specify "6061-T6" or "6082-T6" — not just "6061."

T4 temper: Solution heat-treated and naturally aged. Lower strength than T6, better formability. Rarely the right choice for machined fittings.

O temper (annealed): Fully soft. Used where maximum formability is needed — not for structural fittings.

H temper: Strain-hardened (for non-heat-treatable alloys like 1xxx, 3xxx, 5xxx series). Not applicable to 6061 or 6082.

Step 3: Specify the Thread Standard

Thread standards for aluminium fittings follow the same international system as brass:

• BSPP (G thread): British Standard Pipe Parallel — UK, EU, Asia, Australia. 55° thread angle, parallel.
• BSPT (R thread): British Standard Pipe Taper — UK, India, Asia. 55° thread angle, tapered.
• NPT: National Pipe Taper — USA, Canada. 60° thread angle, tapered.
• Metric (M thread): Common in pneumatic instrumentation and some European fittings. M5, M6, M8, etc.
• UNF/UNC: Unified National Fine/Coarse — USA standard machine threads. Common in aerospace aluminium fittings.

Specify completely: "G ½ BSPP female" or "½" NPT male" — never just "½ inch thread."

Step 4: Specify Surface Treatment

See our detailed anodising guide for the full options. In brief, your specification should state:

• "Bare / uncoated" — only for dry indoor environments
• "Type II anodised, clear, sealed" — general indoor/sheltered applications
• "Type II anodised, black dyed, sealed" — standard for pneumatic fittings
• "Type III hard anodised, 40 μm minimum" — wear-critical or corrosive environments
• "Chromate conversion coating (Alodine), RoHS compliant trivalent only" — aerospace applications requiring paint adhesion primer

Step 5: Specify the Applicable Standard

For machined aluminium pneumatic fittings, ISO 8573 governs compressed air quality (relevant to the system, not just the fitting), and individual manufacturers' dimensional standards apply. For aluminium tube fittings in hydraulic or instrumentation applications, ISO 8434 covers dimensions and performance.

A complete specification example:

Material: 6082-T6 wrought aluminium bar
Alloy certification: EN AW-6082 to EN 573-3
Thread: G ½ BSPP female, to BS EN ISO 228-1, tolerance class A
Surface treatment: Type III hard anodise, 40 μm minimum, unsealed
Thread masking: Threads masked during anodising
Pressure rating: 16 bar at 60°C in dry air service
Manufacturing process: CNC machined from wrought bar (no die casting)

A specification written to this level of detail removes ambiguity completely. You will receive what you specified, or the manufacturer will tell you at the quote stage which element they cannot meet — which is exactly the information you need.