COPV Thread Sizing Guide: M18, 17E, 25E, AN — Engineering Selection
The thread on the cylinder neck is one of the most overlooked design choices on a composite pressure vessel. It controls valve compatibility, sealing reliability, ease of service, and — for aerospace applications — qualification heritage. This guide explains the common thread types, when each is the right choice, and how to specify a thread in an RFQ.
What the thread actually does
The thread on a COPV is the mechanical interface between the cylinder and whatever connects to it — typically a valve, a regulator, or a manifold fitting. Three things have to work simultaneously:
- Mechanical strength — the threaded joint has to hold the test pressure (typically 1.5× working pressure) without yielding
- Gas seal — the joint has to be gas-tight at full working pressure, often using an O-ring or metal seal in addition to the thread itself
- Repeatability — service technicians need to be able to fit and remove the valve repeatedly without damaging the thread
Different thread profiles trade these three differently. Below are the thread types you’ll encounter on aerospace and industrial COPVs.
Common thread types
M18 × 1.5 — the small-cylinder default
Metric thread, 18 mm nominal diameter, 1.5 mm pitch. The default choice for cylinders under ~6 L:
- Compact boss = lower mass, smaller envelope
- Wide range of off-the-shelf valve options
- Standard for hydrogen UAV cylinders, breathing apparatus, lab gas
- Typical pressure rating: up to 700 bar with proper sealing geometry
If your application is a drone, CubeSat, or compact research system, M18 × 1.5 is almost always the right starting choice.
M25 × 2 — for larger cylinders
Metric thread, 25 mm nominal, 2 mm pitch. Used on cylinders in the 30–50 L range where the larger diameter cylinder neck supports a bigger thread:
- Higher torque capacity (more thread engagement, more strength)
- Allows larger orifice valves for higher flow rates
- Standard for the MEYER HDRX-400 (40 L) cylinder
17E and 25E (taper threads)
17E and 25E are tapered threads commonly used on European industrial gas cylinders (welding, breathing air, scuba). The taper provides the seal without an O-ring — the threads themselves wedge tighter as you torque the valve in.
- Very mature, widely available valve ecosystem
- No O-ring to age or replace
- Repeated assembly may damage the thread (taper engagement repeats less reliably)
- Less suited to aerospace where O-ring sealing with traceable elastomer is preferred
3/4″-16 UNF and 5/8″-18 UNF
Imperial UNF threads — common in North American supply chains and certain aerospace heritage programmes. Mechanically equivalent to comparable metric threads but use different valves and tooling.
- Required if your programme uses US-sourced valves or has UNF heritage
- Less common in EU-sourced cylinders
- Direct equivalents: 3/4″-16 UNF ↔ M19; 5/8″-18 UNF ↔ M16
M12 × 1 — small/specialty
Used on the smallest cylinders (~0.25 L and below) where the cylinder neck is too small for M18. Common in research instruments, micro-propulsion, and compact gas-storage subsystems.
Choosing a thread for your application
Three questions usually narrow it down quickly:
1. What valve are you using?
Match the thread to the valve, not the other way round. If your fuel-cell drone uses a specific solenoid valve with M18 × 1.5 inlet, the cylinder thread is M18 × 1.5 — full stop. Don’t over-engineer this.
2. What’s the cylinder size?
Cylinder neck diameter sets the upper limit on thread size. Rough guidance:
- < 0.5 L → M12 × 1 or M18 × 1.5
- 0.5 – 6 L → M18 × 1.5
- 6 – 30 L → M18 × 1.5 or M25 × 2 depending on flow / valve choice
- 30 – 50 L → M25 × 2
- > 50 L → typically larger metric or industrial-style threads, may include flange interfaces
3. What does your supply chain support?
If your sourcing partner stocks UNF-thread valves and your customer’s heritage is US aerospace, fighting for metric will cost you weeks of lead time on every component. If you’re in the EU and using EU-sourced valves, metric is the path of least resistance.
O-ring vs taper sealing
How the thread seals matters as much as the thread profile itself:
- O-ring sealed (M18 × 1.5, M25 × 2 with O-ring boss) — gas seal is the elastomer ring; thread provides only mechanical strength. Reliable, repeatable, but requires periodic O-ring inspection.
- Taper sealed (17E, 25E, NPT) — gas seal is the thread itself, wedging tighter under torque. Robust, no consumables, but harder to disassemble cleanly.
- Metal-seal (some high-purity / aerospace applications) — typically a nickel or copper crush gasket. Highest integrity, single-use seal, expensive.
Custom thread options
If your programme requires a non-standard thread — programme-specific aerospace flange, AN-style fitting, customer-specified profile, oxygen-cleaned variant — most cylinder manufacturers can accommodate it. Custom threads add 4–8 weeks to lead time and require a small one-off tooling charge, so flag this early in your RFQ.
What MEYER offers
MEYER HDRX cylinders are available with the following stock threads:
- M18 × 1.5 (default for HDRX-005, HDRX-030, and most cylinders ≤ 6 L)
- M25 × 2 (default for HDRX-400, 40 L)
- 17E and 25E (industrial gas heritage applications)
- 3/4″-16 UNF and 5/8″-18 UNF (US aerospace heritage)
- M12 × 1 (HDRX-0025 micro cylinders)
- Custom threads on request
The thread option for each part number is selectable when you submit an order on the COPV catalog page. If you need a thread we don’t stock, specify it in the RFQ notes and we’ll quote.
