Gifford-McMahon Cryocooler Part 2 - Building The Cold Head And Making Liquid Air

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This is part II of my project to make a Gifford-McMahon Cryocooler to generate liquid air or liquid nitrogen in my garage with a homemade system. In thie video I'll show how I built the "cold head", which is the part of the system that actually reaches cryogenic temperatures at the very tip. This component is usually connected to the compressor with flexible hoses for the high pressure and return lines and placed inside a thermos or dewar to insulate it and collect the liquid nitrogen or liquid air that's produced.

The cold head is made from a 1-1/2" OD 304 stainless tube with a 0.049" wall thickness. A smaller wall thickness would help reduce axial conduction loss, but the maximum pressure would be limited. The cold tip and the hot end are 1-5/8" copper tube sections brazed to the stainless tube with SSF6 silver solder alloy, and the pipe flange and cap for the hot end of the cold head were 1/4" copper plates custom cut by sendcutsend, although these could be cut on a cheap home CNC router.

The displacer piston is 3d-printed ABS filled with 0.2mm steel spheres meant to be used as shot peening media. It has a separate, smaller piston attached to it which lives inside a smaller cylinder that's used for actuating the displacer using the same pressure source as the main compartment of the cold head. Both the actuator and displacer cylinders have their own solenoid valve pairs.

Temperature drops to about -190 within approximately 40 minutes, after which, air inside the thermos or dewar begins to condense and drip out. With a little over an hour of runtime, i collected around 50ml, although more than half of my yield was lost from soaking into the fiberglass wool and then evaporating out. This would be fixed by having the cold head mounted vertically inside a dewar instead of laying on its side.

There were also some helium leaks through the flat gasket on the cold head which caused it to operate around 300 psi on the high side, but the components could happily run at 400-450 psi if the leak wasn't present, and greatly improve performance. With these changes, I'm very confident that the system could produce over 100 ml of liquid air or liquid nitrogen per hour. The power consumption is also relatively modest for a cryocooler, coming it at about 850-900W.

Here's an (incomplete) list of parts used for the project:

1.505" / 1.625" OD Copper Pipe Section (McMaster 5175K128-5175K115) - Hot End / Cold End
1.500" OD x 1/2" Copper Disc (McMaster 9103K9-9103K91) - Cold Tip Cap
0.545" ID / 0.625" OD Copper Pipe Section (McMaster 5175K123-5175K109) - Actuator Cylinder
Copper Cap for 0.625" OD (McMaster 5520K44) - Actuator Cylinder Cap
500 Psi Solenoid Valve with 1/8 NPT Fittings (McMaster 46365K312) - All system valves
0.2mm Shot Peen Media (Regenerator Mesh) https://www.amazon.com/dp/B096WHSHMY
13x9mm Rubber O-rings (Actuator)
32x25mm Rubber O-rings (Displacer)
Silicone Grease for the Actuator and Displacer Pistons
A 1kW+ rotary compressor (Preferrably for R410a/R32, but others will work)
The remainder of the components are generic refrigeration parts that you can get at any hardware supplier - 1/4" copper tubing, 1/4" flare nuts, 1/4" Refrigerant hoses, needle valves, ball valves.
Copper-Phosphor Alloy Brazing rods for copper-copper joints
SSF6 silver solder rods for stainless-copper joints

Link to further documentation:
https://drive.google.com/file/d/1fwJa_NAICM9mWthuvgpzIsjBWcY405dH/view?usp=sharing

Link to STL files for the displacer/actuator piston:
https://www.thingiverse.com/thing:7404479

Other DIY GM Cryocoolers:
https://middletmakes.com/2026/08/18/diy-cryocooler-and-liquid-nitrogen-generator/
https://www.youtube.com/shorts/CoozzjoCyYc