Transmission line flare nut

[geauxnow]

At a loss for this nut size on my 700r to the fluid cooler. Ive tried 6an, 5/16, 1/2”, 14mm, 12mm and others. One of the lines has the nut; the other line was cut and the nut is missing. What size is it? Rather than replace the line, I’ll barb from a hose to the radiator.
What size is the nut?

 

[Turbo4whl]

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5/16" inverted seat to 5/16" hose barb

Dorman

If the steel tube still has the flare, then the nut just slid down the tube, it may still be there.

 

[geauxnow]

Thanks. I’ll give it a shot this weekend.

 

[geauxnow]

You nailed it, @Turbo4whl thank you