On Sun, 14 Jun 2009 10:35:18 -0400, BQ340
wrote:

gary wrote:
I want to recharge the refrigerant in my auto air conditioner. I've
assembled all the necessary tools except for one item. My vacuum pump
is outfitted with a 3/4" female NPT fitting and the discharge tube
from the gauge set is a R134a female fitting. I plan on machining an
adapter to get the 2 fittings connected to each other.

But I can't figure out what the thread size is on the R134a. Please
enlighten me!

1/2" Acme as I recall but I don't remember the pitch, sorry!

Don't reinvent the wheel by machining the fittings, it's silly to
waste $100 to $200 of your time on a $5 part - you only do that when
it's made out of pure Unobtanium... Go to any refrigeration supply
house or search online.

They have in-stock adapters to go from NPT pipe thread to 1/4" SAE
Flare (HVAC and Refrigeration charging hose fittings) 3/8" SAE Flare
(manifold to vacuum pump hose connection) and 1/2" Acme Flare (R-134a
Automotive Manifold hose fittings)

Your vacuum pump should have 1/4" SAE Flare and 3/8" SAE Flare
fittings if it was made for refrigeration work - you connect the
manifold hose with the 3/8 fitting, and can attach the micron vacuum
gauge to the 1/4" to monitor your progress.

(If the system under vacuum has an extra service port, connect the
vacuum gauge sensor directly to it. That way you can isolate the
system with a ball valve and shut off the vacuum pump, and monitor the
vacuum levels for leaks and/or trapped moisture still evaporating from
the refrigeration oil.

If you are adapting an industrial vacuum pump, you'll have to
bushing down from 3/4" NPT down to the threads needed for the adapter.

-- Bruce --