I started into the 622 compressor "rebuild". I forgot to start taking pics, so I figure there's enough videos and crap out there showing what you need to do, so I'm not going to bother. But I will share here a little on what I've learned so far between short shaft and long shaft compressor guts.
GM used about 4,000 different part numbers for R4 comprressors and compressor parts, depending on whether there was a superheat switch or pulley arrangement, shaft seal types, and shaft lengths.
This makes it a MOTHER bear to try and figure out what they're doing. Going strictly by part numbers and supersessions, it can get you in trouble real quick as you have to figure out where else to mix and match.
R4 compressors utilize what's known as a scotch yoke to reciprocate up and down and side to side movement on the "crank shaft" all at the same time. What this does to the 4 pistons is act like the pistons are doing "the wave" around a circle, if that makes any sense. Made to reduce the pulsations of an otherwise piston pump. The pistons are hard pressed into the yokes, and separating them usually proves disaterous, usually ruining the cylinder guts. This is why they come pre-assembled and the only replaceable parts for them are the discharge valves held on with inside snap-rings.
For one, the shaft length between my R4 compressor "cylinder kit" and the existing 622 is the long shaft. The 622 compressor is the short shaft. Otherwise, it's the same exact guts inside. In fact, most all R4's have pretty much the same rotating guts other than shaft length. They use the same outlet valves, pistons, internals. Again, only the shaft length and whether or not it has provision for a superheat switch or not stuck on the back of them seems to be the determining factor. 622 (short shaft) and 198 (long shaft) are identical aluminum cylinder blocks which do not use the switch as there is no provision for one, and the only difference in the rotating assembly is the shaft length. NOTE: If you have a cylinder block that has a plug in the superheat switch but is otherwise useable, it's ok, you can just leave the plug in place. Won't hurt anything.
The long shafts are about 19/32" longer, thereabouts, than the short shaft, from what I can measure. They also used o-rings for sealing the inlet/outlet ports, where later they used washers and the 134a started using the mismatched heights in the ports. For anyone that's ever done R4 re-seals or work on one, you know the cylinder kit consists of the cylinder block, pistons, crankshaft, and scottish yoke assembly. It normally comes with an O-ring and shaft seal kit and new thrust washers in the box. It's basically missing the shell, outlet valves, head unit (where the front bearing and shaft seal resides, and the clutch/pulley junk.
What this does, however, is move the keyway at the end of the shaft further out. And while the 622 and 198 share the same magnet, pulley, rotor/bearing, the clutch assembly itself, which presses onto the shaft is a tad different in length. The 622 is a bit longer while the 198's is a bit shorter. Same keyway and everything. I'm going to go out on a limb here and while I didn't measure, I suspect the 198 clutch plate shaft is about 19/32" shorter than the 622 clutch plate to make up the difference in shaft lengths. What is different also is that while the 622 has metric inner threads used to pull the clutch plate assembly off the shaft, the 198's has SAE threads. So watch out for the puller usage. This is for GM/ACDelco parts only. I cannot attest to any aftermarket parts. So in theory, you should match short shaft with the longer reach clutch plates, and long shafts use the shorter reach clutch shafts.
So while the part number for the cylinder kit I have has a supersession number which also corresponds to one fitting both 198s and 622s, there is that little issue with the clutch spring plate in the front. At minimum, I would have to resurface the rotor/bearing and clutch plate since the 622 turned blue from the lockup. It didn't take long. I may be able to resurface it, but since I already have new GM parts, it's easier to replace...I haven't got there yet, but I've got this sneaky suspicion I'll have to use the 198 style clutch plate in front.
Another interesting note about the front head unit that bolts onto the front with 4 bolts, is that the 198 and 622 have different part numbers. 198 has GM p/n 6551142 and the 622 uses GM p/n 6552240. I don't know the exact difference of what that is, but in 1988, the 198 head unit p/n was superseded by 6552240 anyway. So I went ahead and changed the front shaft bearing out (S-128 or B-128 is the standard bearing number in case anyone is wondering, or GM p/n 6556572) since the original compressor blew up and I didn't trust metal bits not getting into the needle bearing. Additionally, 80s style front head units have an oil hole that goes on top to direct oil into the bearing area for lubrication. The housing is marked with a little X molded in to designate the top to ensure it's put back in the proper position.
I think what I'm going to end up with is that I'll technically turn a 622 compressor into a 198. Which is fine, because IMO, there's really not a lot of difference. I think it was more along the lines of the advent of the polygroove pulleys which utilized the short shafts. There's also a lightweight R4 as well, but I really don't know much about those as far as part differences. Besides, I can put a Harrison decal on it and unless you know what you're looking at, nobody would be able to tell otherwise that it is or isn't what the decal says it is. I'm also using the ACDelco 89019204 double lip seal vs. the standard single lip seal Delco replacement. Apparently the double lip seal provides static leak protection when the compressor sits along with the rotational leak protection. The regular Delco single lip seal provides good rotational seal, but not so much on static. The shaft seals and shell o-rings are a common cause for R4 compressor leaks.
Additionally, when reassembling an R4 compressor, use the R12's MINERAL OIL for lubing the pieces parts. While that may seem contradictive if you're going to use 134a or even 152a, it's ok. Just don't need a ton of it. Just for lubing O-rings and such. It won't mix with other refrigerants too well anyway, but the main thing is, especially if it sits for a little while before you use it, it is not a hygroscopic (water absorbing) lubricant like ester or PAG. So if you sit it on the shelf for a bit, the ester/PAG oil is sucking up moisture right to those seals you want to prevent rust. Not a great idea. So use a bit of mineral oil for reassembly.