O-Rings are a pain. Servicing AC systems that don't use them is so much simpler, but I guess thay are here to stay.
There is some confusion concerning which O-Rings are suitable for R12 and R134a and whether they need to be changed when changing refrigerants.
The standard O-Ring that has been in use for 50 years or more in hydraulic systems and AC systems is known by various names such as NBR, Nitrile Rubber, Buna-N, or just Buna. They are usually black, but could be any color. The standard NBR O-Ring has just a fair to middlin' reputation in most hydraulic systems because it was originally designed to operate with aircraft hydraulic oil which contains an additive such as tri-cresyl phosphate that causes the NBR O-ring to swell just little and therefore increase its sealing ability. With typical oils used in marine and industrial hydraulic systems, NBR O-Rings usually shrink a little and may cause leakage problems. Compatibility of NBR O-Rings is classed as good with R12, R134a, Mineral oil, POE oil, and PAG oil. The temp range for NBR is -22F to 210F.
The marine and industrial systems users answer to the O-Ring problem was (and still is) the premium O-Ring compound Viton, which is a flourocarbon rubber compatible with mineral oil and many other hydraulic fluids. Viton O-rings sometimes are brown, or black with a blue stripe, or just black. The temp range for Viton is -4F to 392F.
Viton is not compatible with either R12 or R134a. You won't find it it any factory AC system and you shouldn't use it for replacements.
A better type of NBR O-ring is the HNBR which stands for Hydrogenated NBR, and is sometimes called Hydrogenated Nitrile, or Highly Saturated Nitrile Rubber, or HNB. The compatibilty of HNBR with Mineral oil, PAG, POE, R12, and R134a is about the same as NBR (good). Its big advantage is that its temperature range is -22F to 302F, which is a big improvement on NBR for those areas that need the extra heat resistance. The switch to HNBR is really only for heat resistance, and maybe a little abrasion resistance for the Ford springlock couplers; it is not a chemical compatibility issue. NBR will work just as well in the lower temp areas of AC. HNBR are frequently green, but they can be black also, or other colors.
Getting the correct O-Ring can be a chore. They are also shelf life items that go bad with time. The rule in Military systems is that an O-Ring is prohibited from being used unless it is taken from an individual package that has the correct part number, rubber compound, and cure date, and expiration date imprinted on it (with the expiration date not passed, of course). This is clearly impractical in most fields, but it shows that when you buy an O-ring, you may never be sure what you are getting, because you can't always go by color, and you don't know how old it is. It is best to deal with a reputable, high volume supplier.
The compatibility of NBR and HNBR with the majority of non-HC alternative blend refrigerants is marginal to poor. The blend suppliers are recommending a change to Neoprene O-Rings for their blends, and the problem is that neoprene has even less heat resistance than NBR. That's another reason the stick with R12 or R134a.
Edited: Sat September 27, 2003 at 5:50 PM by Mitch