Equipment Project Completed: Atlas Copco Air Compressor

The Gorton Mill thread has been a success, so let’s try it for our other ongoing projects too.

The Atlas Copco GX4 was brought to the space shortly before Covid-times and stalled out for lack of attention during lockdown. We’ve taken a look at it and think there’s not much left before we can bring it online, but there may still be hidden problems. What’s left to do?

  1. The tank drain valve is missing, so it can’t build pressure. There should be an auto-drain here eventually, but a temporary manual drain valve would be sufficient to unblock the project.
  2. The air pressure switch is on the high side of the air/oil separator assembly rather than on the tank like it should be. There’s a 1/4" air line that it should be on, and the connection on the oil separator should be capped.
  3. There may be a problem with the minimum pressure valve in the air/oil separator, but we can’t really tell until we can run the tank up to pressure.
  4. The automatic filter cleaner solenoid is missing. Not a dealbreaker, but should be fixed eventually.
  5. The refrigerated dryer needs to be thoroughly checked out, including the automatic condensate drain. The compressor runs and there’s some cooling to the touch, but we should find the specs and confirm it’s running the expected ∆T and draining properly.

1, 2, and 3 need to be resolved before we can press the Copco into service and retire the tired, tired, Husky piston compressor.

Project wiki page, where manuals etc will be shared: Atlas Copco GX4 Air Compressor - Asmbly Wiki


A 1/4" air line that had been added where the emergency relief valve is supposed to be, I think that may be item #2 there. I got a new relief valve and installed it in the port.

I went back through and the primary problem is that part of the thermostatic valve is still missing. I obtained the whole kit for servicing the thermostatic valve and MPV, but that kit is only maintenance items- springs, seals, and the thermostat element. It does not have the cylinder the thermostat actuates, and Atlas Copco parts diagram graphically depicts this part but does not have a part number on it. Stepping outside the parts diagram, I searched online and there are several similar AC thermostatic valve actuator cylinders, some specifying which model they go with- never the GX4 though- and several with a unique AC inventory number but no AC model number associated with them. So, one of these is probably the correct part but that documentation is somehow not publicly available on that specific part.

I went back to AC parts department and they assured me they can supply the part (or clarify which part number this is), I sent them detailed info last week and am waiting to hear back.

There may or may not be parts missing from the MPV as well- the parts diagram’s details are a bit hard to make out and correlate with the plug piece- it seems to be an assembly that comes out as one piece, so it actually may contain all the parts in the diagram. Again, I detailed this with our AC parts guy late last week and waiting for more info. I already pinged him again today. He may need to dive a bit deeper to find this info since his first reference is probably the diagram we’re already looking at with no part number on that part.

The oil sep manifold does seem to be building up excess pressure without going into the tank. However, I have doubts this is a missing part in the MPV assembly, as that would likely leave the MPV open, not closed. It could be the oil sep filter is clogged. We have a new filter but it is not installed. The oil sep filter is a separate thing than the oil filter, both look just like automotive oil filters though. We have both on hand. But the oil sep filter lacks grip surfaces like an automotive oil filter and we couldn’t figure out how to remove it. I think a strap wrench would do it.

I think I have an auto-drain valve that will fit this. I can drop it off soon.

Given that the thermostatic oil valve assembly is entirely removed, and possibly a part from the MPV assembly, I don’t think we should try to power it up or build pressure until we have more guidance from Atlas Copco. This could deprive the screw of oil, and things will probably not make sense with these parts removed. Also the plugs are not tightened down when I last finished documenting the issue, so it may spray oil out if pressurized.

But, the screw compression unit itself and the motor appear to be in good working order, those are the big parts, so it looks pretty great overall. It hinges on getting that part info from AC part department, and I think they’ll be able to provide that soon and we can move forward.

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I got a nice part diagram from Atlas Copco via the serial number. It is the correct one

It’s what Atlas Copco has on the books, but it’s not what it’s supposed to be. It’s the thermostat but not the valve assembly.

I finally think I have found the correct part for the thermostatic valve and ordered it. Boy, that was harder than it needed to be

I’m marking this project completed. The Copco is hooked up and supplying shop air. Thanks to everyone who helped!