You can take the valve covers off without any concern by removing the four bolts at the corners of each cap.
All sound advice, but I'll add this part - you can easily break a valve cover plate by
removing those four bolts while the valve is tensioned either by the set screw or the threaded ring/bushing. Best practice is to de-tension the valve by loosening the set screw/threaded ring, then remove the valve cover plate.
You have to be careful putting them back on. On caps with unloader towers, remove the tower by unscrewing it and then make sure the threaded ring is backed off so that it doesn't contact the valve body when torquing down the caps. Likewise, back off the set screws which retain the valves in the caps with no unloaders. The caps must be fully bolted down first, then the retainer rings and set screws are torqued down to hold the valves in place inside the head, then the unloader towers and large hex nuts are scewed on last. If you don't back all of these out before reinstalling the valve covers, you can split the covers when you tighten the cap bolts.
100% in agreement - a torque wrench is a critical tool for rebuilding a QR pump! There's a reason for the various torque specs, and it's not as an inconvenience. I can't stress enough how easy it is to split those valve cover plates. I recently rebuilt a QR-25 model 350 with a few friends and when we got past all the grime and build-up on the outside of the head, we found one valve cover plate that was brazed back together as well as an unloader tower that was brazed to its plate. Obviously, neither valve was tensioned correctly and the head was absolutely full of carbon chunks. They had also routed the pneumatic and hydraulic unloader lines from the pilot to the towers incorrectly such that it was sucking up oil from the crank case and depositing it into the head, then blowing it into the tank. They kept adding oil (probably 10w30 - incorrect) to keep it running, but the problem accelerated when the tank got so full of oil that it was just siphoning oil from the tank and crank case back into the tank. It was a clever [sic] arrangement that eventually kept them from using the pump. Interestingly, the guy who sold it said "Oh, it doesn't need to be rebuilt, she ran just fine last time she was on."
Very unlikely given at least two (of 6) valves weren't seated whatsoever.
The retainer rings look like short pipe *******. There are two notches in the top to insert a special ring wrench which has two internal square teeth to engage the notches so you can back the rings out and tighten them back up. Most people do not have access to the ring wrench, so you can use a piece of key stock in the notch and use a pipe wrench or an adjustable wrench to turn it. The set screws and rings may be difficult to turn if there s rust. Take your time and use lots of penetrating oil.
Mild steel flat bar welded across the flats of a large socket also works well!
The valves themselves are pretty simple affairs. They just unscrew apart. Don't lose the plungers or springs for the unloaders. You can lap the valve seats on a piece of glass with some wet and dry paper on it. Use a figure 8 motion and some lube. After you reassemble the valves, fill them with solvent and see if they hold it, or it leaks out. That will show you if they are in good shape. You can heat up and quench the copper ring gaskets under each valve, but generally all they need is a clean up and reinstall.
I personally use a surface inspection plate with 600-800-1200 grit wet paper + lapping compound on the valve seats and have had excellent success. I believe the valve rings are through-hardened, but haven't tested that (yet?) to confirm. I usually take them to the 800 grit with lapping compound to make sure they're absolutely flat as well - it's not often I find a new one that needs more than a few rubs at 800/1200 to be honest. The result may be a seal that's tighter than the original that came with the compressor.
At any rate, as soon as we settle from our move, I'm going to start a QR-25 model 5105 rebuild that's going into my garage. I and a friend took a small 3 hour trek and grabbed it for a song and a dance and it's practically mint on the inside by all initial indications. I'm going to document it and an X8 rebuild on GJ, perhaps that will help others along with what I've learned <not> to do.
Good luck, OP, you've got one of the finest!
