you seem to know your stuff!!! On the topic of this, i want to continue using my original R4, any idea for some quality companies i can send it too for rebuilding?
I wish I knew more about these A/C systems.
Nobody I know specifically rebuilds the R4s as a side job. In other words, I know of nobody simply saying "send me your original R4 and I'll restore it." There's some R4 gurus out there, for sure, but nobody is in the business of rebuilding yours that I'm aware of.
If the rotor and pistons are hosed up, it's pretty much done. Replacing it is cheaper. Used to be able to buy Delco aluminum cylinders/piston assemblies, but they've pretty much dried up or unless you spend some $$ to which, buying a rebuilt unit could be cheaper if you're not R4 savvy. There's so many different part numbers and supersessions it's tough to figure out what fits and what don't. GENERALLY speaking, R4 compressors in the same year span tend to be approximately the same. Not all, but most. For example, some systems like a lot of Chevys and some others have a high pressure switch, etc., located in a hole in the back of the compressor, setpoints roughly at 425 psi give or take and reset around 200 psi. This cuts off the compressor if pressures get too high on the high side. They insert plugs for those compressors that don't require a switch sometimes, so you can remove the plug and install a switch if needed. But you cannot add a switch to those that didn't come with a provision for one. They have a molded over indention. Two that come to mind are the Oldsmobile 198 and 622 compressors without a provision for a high pressure switch.
Here's a compressor with a high pressure switch provision and just has a plug. Can be run with or without the high pressure switch requirement. The clip and plug comes out, and the switch can be installed.
And here's one without a high pressure switch provision. Most Olds V8 applications came with the 198 (long shaft) or 622 (short shaft) compressors, and no provision available from the factory. This is my 622 fried compressor to which I replaced the guts with a new Delco cylinder assembly and made it into a 198 with new upgraded shell O-rings and Delco double lip shaft seal and nearly all new Delco parts up front. I reused the shell, pulley, and pulley perimeter bolts, though.
Switch or no, these are the two main types of compressors. Essentially, they all work the same. The magnets and hubs and pulleys and such change, but mostly the R4 is an R4. The rotating assembly is the SAME. Length of the shaft may vary a little, but not by much. Also, later ones that were actually used for R134a had a step in the suction/discharge ports that matched up to a specially designed R134a hose, such as on the 93 Camaros and I believe up through mid-90s trucks.
Whether they call them heavy duty or light duty, in reality, there's regular R4, and light-weight R4. The lighter weight comes from the ligher pulley and magnet assembly upfront on newer models. But G-body fans need not worry. They ALL got the regular R4s if they got R4s and the separate V-belt. I believe the serpentine belts used the lightweight version, like the 86-87 GN. But mostly they used "regular" R4s in the 80s.
The rebuilders that are assembly line rebuilt normally use cheap azz parts and they usually don't last long. Even Four Seasons. Not always, but mostly. Sometimes you find a gem on a rebuild, but not often. It's a crapshoot. The main key to R4 compressors is sealing the cylinder casing to the outer shell with the two main o-rings as they fail a lot, as well as the single lip seal in front. A double lip seal works better. If the bearings are worn, they don't care and rebuild them anyway. Sometimes they last, sometimes they don't.
Again, I always stress to cycle any R4 compressor that's been sitting idle for a good while. Especially new ones. Reason is the pistons on their lowest point in the stroke expose part of their sealing rings for some stupid reason. And they are compressed while inside the bores, but will expand enough to prevent the assembly from rotating if exposed and left alone long enough. You need to gently bump the rotating assembly to get the exposed piston seal to go up inside the bore. Then, let it sit for a day or two, then you should be able to rotate it so much easier. If you do that periodically, like at least once every 6 months, you should never have a problem once you mount it (and run the compressor occasionally). And don't use the nut on the end of the shaft to turn it or you'll cry when you strip the threads. The nut doesn't do much but is a backup retainer for the friction-fit clutch plate. Use a tool for rotating the clutch plate to turn the rotating assembly. Kinda tough to do but it can be done. Ensure there's a bit of mineral oil in the compressor for lubrication since it's aluminum and you don't want to score them. You can drain any excess out later. Residual mineral oil in the compressor will not affect any conversions if you're not using R12. And for goodness sake do NOT let any dirt get into the cylinders if they are exposed to the environment during a re-O-ring job. Clean clean clean is the words of the day.