NFPA vs. ISO Cylinder Standards — Which One Is Your Machine?
Dimensional differences, bore/stroke cross-reference tables, mounting style comparison, and a quick-reference decision guide for specifying the right Aventics series.
Why This Matters
Specify the wrong standard and the cylinder does not fit. The mount holes miss, the port threads will not seal, the machine cannot be re-sourced, and the line stays down while someone re-engineers a bracket that should never have been needed.
The trap is that the two standards produce cylinders that look interchangeable in a catalog thumbnail and behave nothing alike on a machine. Bore and stroke are the numbers a buyer fixates on, but they are the least of the fit problem. What strands a replacement is the mounting bolt circle, the rod thread, the port thread form, and the overall envelope — all defined differently by each standard. An engineer who confirms only the bore has confirmed almost nothing about whether the part will bolt up.
The Fundamental Difference
NFPA is the North American, inch-based standard (NFPA T3.6.7 and T3.6.8). ISO is the global, metric standard (ISO 15552, and ISO 21287 for compact cylinders). Both define bore sizes, rod diameters, mounting interfaces, and port locations — but in different unit systems and different dimensional series. They are NOT interchangeable. A cylinder built to one standard will not bolt into a machine designed for the other without adapter hardware.
Bore Size Cross-Reference
The nearest metric equivalent to each inch bore is close enough to confuse a parts clerk and far enough to fail in mounting. Use this table to translate, not to swap:
| NFPA (inch) | Closest ISO (metric) |
|---|---|
| 1.5” | 40 mm |
| 2” | 50 mm |
| 2.5” | 63 mm |
| 3.25” | 80 mm |
| 4” | 100 mm |
| 5” | 125 mm |
| 6” | 160 mm |
Mounting Style Cross-Reference
Mounting codes translate roughly between the two systems, but the dimensions behind the codes do not match. Treat this as a "find the analogous style" table, then confirm actual dimensions:
| NFPA Mounting | Approx. ISO Equivalent |
|---|---|
| MF1 — head rectangular flange | MT4 |
| MF2 — cap rectangular flange | MT3 |
| MS2 — side tapped | MT1 |
| BB — foot mount | MT2 |
| CB — cap clevis | MT7 |
How to Identify Your Existing Standard
You can identify the standard from the cylinder itself in under a minute. Check these four things:
- Body and head style. NFPA cylinders have square or rectangular heads with tie rods and inch-marked bore and port sizes (NPT/NPTF threads). ISO cylinders use a smooth profile body or tie rods with metric bore and port sizes.
- Port thread. NPT or NPTF thread means NFPA. BSP / BSPP (G-thread) means ISO. This is the fastest single tell — put a thread gauge on the port.
- Bore marking on the barrel. Markings such as 1‑1/2”, 2”, or 2‑1/2” indicate NFPA. Markings such as 32 mm, 40 mm, or 50 mm indicate ISO.
- Part-number prefix. In the Aventics catalog, NFPA cylinders carry an "A" prefix; ISO 15552 cylinders are the 450 / 452 / 453 Series.
Aventics Series by Standard
| Aventics Series | Standard | Bore Range | Max Pressure |
|---|---|---|---|
| NFPA Series | NFPA T3.6.7 | 1.5” – 14” | 150 PSI |
| S Series (Stainless NFPA) | NFPA footprint | 1.5” – 8” | 150 PSI |
| 450 Series | ISO 15552 | 32 – 125 mm | 10 bar |
| AV Series | ISO 21287 (compact) | 20 – 100 mm | 10 bar |
Two dimensional details deserve attention when you cross between series. First, rod thread: NFPA cylinders use inch rod threads (UNF), while ISO cylinders use metric rod threads — a rod clevis or coupler machined for one will not thread onto the other. Second, port thread: NPT is a tapered thread that seals on thread engagement, whereas BSPP (G-thread) is parallel and seals on a bonded washer or O-ring at the port face. Forcing an NPT fitting into a BSPP port leaks or cracks the boss. Budget for new fittings and new rod hardware, not just the cylinder body.
Field Scenarios — What to Spec
- "Replace a cylinder on an older American-built machine." NFPA. Confirm bore in inches, port type (NPT), and mount code.
- "Designing a new machine for export to Europe." ISO 15552, metric throughout.
- "My machine was built by a European OEM." Almost certainly ISO. Check for G-thread ports to confirm.
- "I need a food-zone replacement." Aventics S Series stainless (NFPA footprint), or a standard ISO cylinder with FKM seals if the machine is metric.
- "I need a compact cylinder in a tight space." ISO 21287 (AV Series) or an ISO 15552 compact variant.
Quick Decision Table
| Situation | Standard | Aventics Series |
|---|---|---|
| North American OEM machine | NFPA | NFPA Series |
| Food / washdown, US machine | NFPA stainless | S Series |
| European OEM machine | ISO 15552 | 450 / 452 / 453 Series |
| Compact, space-limited | ISO 21287 | AV Series |
| Export machine, metric | ISO 15552 | 450 / 452 / 453 Series |
Free Cross-Reference Service from Air-Oil Systems
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