We bought it knowing it had a 'small knock' and after taking it apart and seeing no obvious cause decided to take the crank and rods to the machine shop to have them inspected. The machinist (who has always been exceptional) found that the last time the unit was machined it was done so incorrectly resulting in a taper in the crank journal. He was very confident this was the source of the knock and that there was enough meat left on the rod bearings to re-machine it again. We had him do so and despite it taking several years for us to get back to reassembly we were confident the compressor was now a runner. It was clear from other parts inside (new bearings and seals) that the unit had not been run long after the last rebuild. This fit with our 'machined wrong' theory. You can see our confusion and dismay in the video when it still knocks at the end. There is no journal or wrist pin slop that we can see and all we can figure is piston slap on the larger first-stage side. We will try spinning the compressor up to see if pressure on the piston will hold it straight and stop the slap but it looks like it might be time to put this war horse down. Very disappointing. We welcome input from those who might have other ideas or have worked with these compressors themselves.
We are not yet sure if the unit is serviceable but the video is still a good look at how you would go about rebuilding a compressor of similar make and size.