Every time I find myself needing this information, searches turn up wrong or incomplete information. What are the standard sizes for automotive air conditioning service valve and hose fittings for different refrigerants, and how are they labeled for the sake of buying matching adapters, hoses, manifolds, vacuum pumps, etc.?

I'm most familiar with R12 where the sizes are 7/16" (low pressure) and 3/8" (high pressure), but I've also seen the "standard manifold hose" sizes described as 1/4" and 3/16". Is this just a difference in what's being measured (thread diameter vs hose diameter?) or is something else going on?

Ideal answer would be a table of each type of connection point (low/high pressure side, type of system or device(s) it's used with, etc.), what name(s) it goes by, and the physical dimensions the name is referring to.

  • The A/C service machines owned by friends of mine are supplied as standard with several different adaptors to be able to deal with most cars on the market... what machine do you have?
    – Solar Mike
    Aug 7, 2018 at 17:38
  • 1
    Great question. Aug 7, 2018 at 17:54
  • @SolarMike: I don't have any machine; they're really way out of the reasonable price range for doing work on your own vehicles. Emptying system for current work will be doing by pulling a vacuum with a very cold reclamation tank. Aug 7, 2018 at 19:06
  • Early R12 had same size fittings low and high (3/8), later years had standard fitting for low but changed fitting (7/16) for high side for safety reasons.
    – Moab
    Aug 7, 2018 at 19:10
  • @R.. It's difficult to know that it was for "personal use only - having that in the original question would have been helpful.
    – Solar Mike
    Aug 7, 2018 at 19:44

1 Answer 1


Based on everything I've found buying tools and adapters:

The R12 low pressure size is the same as the "standard manifold hose" size, and is normally called 1/4, though it's clearly larger than 1/4". It seems to be a "National Pipe Thread" size, which from my experience with plumbing, is a huge illogical mess.

Because of the pipe wall thickness of Schedule pipe, the actual diameter of the NPT threads is larger than the Nominal Pipe Size outside diameter, and considerably so for small sizes.

According to the linked article, the tap drill size for "1/4" NPT is 7/16", which probably explains why I've seen it called 7/16 before.

The R12 high pressure side seems to be called "3/16", and is likewise mismatched with the actual physical size.

This answer is not complete, and I would still love to see the kind of answer sought in the question, but I figure it may be useful to anyone who finds this question and is equally confused until there's something better.

Update: It's actually SAE 45° flare sizes, detailed here. Like NPT, the nominal sizes are sizes for an assumed inside of the pipe, not the size of the thread, and need a conversion table. Basically, adding 3/16" for small sizes, 1/4" for medium sizes, and 3/8" for large sizes. For the sizes used in AC service valves, they match the original answer above, and the thread pitches are 20 TPI for low pressure nominally 1/4" and 24 TPI for high pressure nominally 3/16".

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