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Help with fittings/adapters

MikeC5 on Sun July 26, 2009 6:20 PM User is offlineView users profile

Year: N/A
Country of Origin: United States

I was given an older Welch vacuum pump that seems to run well and I'm going to try and adapt it for AC use. The only problem is that the threads for the outlet are pipe thread and I'm having trouble trying to find an adapter or combination of adapters to get the yellow hose hooked up. I'm reasonably certain the pump outlet is tapped for 3/8 NPT. I'm not sure what thread type or size is used on the gauge hoses. I know they are 90 degree flare but the threads? I have looked through McMaster-Carr but don't see anything to get me from 3/8 NPT to standard (R-134a) fittings.

Cussboy on Sun July 26, 2009 9:31 PM User is offline

You can go to 1/4 inch male flare (like used for R-12). Then get the Acme adapter from board sponsor. These are used to convert older vacuum pumps from R-12 to R134a; I have such an adapter.

82031 - Vacuum Pump Adapter

Edited: Sun July 26, 2009 at 9:33 PM by Cussboy

MikeC5 on Mon July 27, 2009 8:48 PM User is offlineView users profile

Interesting... my new, (supposedly R-134a) gauge set uses 1/4 inch male flare for the hose hookup on the gauge high and low pressure ports. The yellow charging hose hookups are 3/8 male flare. What size are they supposed to be on a gauge set for R-134a? I did find a 3/8 pipe thread to 3/8 flare adapter at Home Depot (of all places).... Thanks for the help.

Chick on Mon July 27, 2009 9:23 PM User is offlineView users profile

What is the year make and model of your car..It might be an R12 system....

Email: Chick


Freedoms just another word for nothing left to lose

MikeC5 on Tue July 28, 2009 6:00 AM User is offlineView users profile

It's a 95 Subaru- definitely R-134a. The gauge set uses the 1/4 and 3/8 male flare fittings...

mk378 on Tue July 28, 2009 9:52 AM User is offline

A true 134a auto gauge set is acme thread throughout, especially on the yellow hose. Cylinders and top can taps of R-134a found in the auto market have an acme male outlet so you will need to adapt to that in order to be able to charge a system. If you can find a side can tap or cylinder of R-134a sold for other purposes it will be 1/4 inch flare.

MikeC5 on Tue July 28, 2009 8:24 PM User is offlineView users profile

I think I made a Junior Member mistake... Flare fittings apparently look a lot like ACME fittings to the untrained eye.... so this isn't so easy after all. The male fitting on the gauge set for the evac hose (yellow) is ACME I think. The threads on the fitting come out closer to the conical end where the sealing is done (compared to a regular flare fitting).. is this correct? So when I re-read Cussboy's post, getting an adapter to go from 3/8 pipe thread to 1/4 inch flare is readily available but then you have to go from 1/4 inch flare to ACME...

CCWKen on Tue July 28, 2009 9:07 PM User is offlineView users profile

First of all, are you sure it's 3/8" pipe? That's about a 1/2" diameter fitting. That's big for a manifold. Most are 1/4" pipe. The 1/4 pipe to Acme adaptors are available. I used a couple when I added a Tee to my manifold. It allows me to use two hoses on the input side (vacuum/refrigerant).

Ken Kopsky

Custom Car Works
"Rules are for the guidance of wise men and the obedience of fools."

MikeC5 on Wed July 29, 2009 7:52 PM User is offlineView users profile

I'm sure the vacuum pump threads (in the pump casting) are 3/8 NTP. It actually measures about 5/8 diameter which is 3/8 NTP (very logical I must say).. I did order a 1/4 flare to 1/2 ACME adapter and will use that with a 3/8 NPT to 1/4 flare adapter. Not too elegant but it should work.

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