Connect Industrial / Milton M-style air coupler to 1/4" BSP parallel (G1/4 / BSPP)
How to join 1/4" Industrial (Type M) and G1/4, and what stops it leaking.
Not a direct fit. Joining Industrial / Milton M-style air coupler to 1/4" BSP parallel (G1/4 / BSPP) needs a 1/4 BSP air coupler on a G1/4 port.
The part to buy
Most 1/4 inch air couplers screw onto a G1/4 (1/4 BSP) port on a tool, hose or manifold, sealing on PTFE tape or a bonded seal. The coupler profile (Industrial, ARO, Euro) decides what plug fits the front; the BSP thread decides how it mounts. Pick the coupler profile first, then the port thread.
The BSP port and the coupler profile are two separate choices. A correct G1/4 thread still leaves you with the wrong front profile if you do not match the interchange as well.
Related connections
Sources: Air line couplers and plugs interchange guide, ToolGuyd · Common pneumatic quick-connect interchange profiles (Milton, ARO, Euro, Orion), XHnotion · ISO 228-1 (G / BSPP parallel pipe threads) · BSP thread dimensions chart (ISO 228 / ISO 7), AmesWeb