Transmission cooler to radiator fittings, NPT or Inverted Flare

[fryeguy2]

I am tired of tranny cooler leaks and am running braided stainless lines wth an fittings for my turbo 350 tranny cooler and aftermarket cooler. I have the proper fittings for the transmission (1/4 Straight pipe thread) but I keep searching the internet and people don't seem to agree on what fittings thread into the stock radiator transmission cooler.
These are the 3 ive been seeing

1/4 NPT
1/2 inverted flare
5/16 inverted flare

Does anyone know for sure which of these is right?

[gastrader35]

did mine the same way the fittings are 5/16 inverted flare

[Billhilly]

Not trying to confuse the issue, I just bought these and they fit mine.
http://www.aeroquip.cc/fcm2963-fbm2963.aspx

[fryeguy2]

Finally got this figured out.

Stock tranny cooler lines use 5/16 hard lines, the fittings are called 5/16 inverted flare fittings because they are made to slip over these lines but the size of the thread on these fittings is 1/2-20. It looks like different manufacturers use these numbers interchangeably calling the same fitting a 5/16 inverted flare fitting and a 1/2 by 20 thread count inverted flare fitting. Making the situation even more confusing 1/8 inverted flare fittings have 5/16-24 threads and 1/2 inverted flare fittings have 3/4-16 threads.

But wait theres more!! There are manufacturers that sell 1/4 NPT fittings that they say are made to thread into the tranny cooler. Looks like pipe thread sizes dont have anything to do with their physical dimensions and 1/4 pipe threads actually measure out at 1/2 inch. So 1/4 inch pipe fittings will screw into the tranny cooler and evidently still do a good job sealing but dont technically have the proper inverted flare on the end that seats against the flare on the tranny.

So basically all three choices seem to be right, 5/16 inverted flare fitting which should have 1/2-20 threads, same fitting is often called a 1/2-20 inverted flare fitting, and 1/4 NPT seems to work fine as well.

[eagleuh1]

5/16 is the tubing size, correct. At least on my 68 thats what it had.

[Shane]

NTP and tube fittings have nomenclature that is arranged according to the size of the tube that it connects to and NOT the size of the thread.