AOD Transmission cooler lines

[jamesroney]

1/4” NPSM: 18 threads per inch, OD measures .517"-.526"

Note that straight pipe threads, unlike tapered pipe thread do not provide any sealing function. Typically, a design that utilizes a straight pipe thread will incorporate a sealing features, like an oring to facilitate fluid sealing.
View attachment 902101

Correct, and if you look at the factory 1/4 NPS to 5/16 SAE inverted flare fittings, you will see a shoulder machined into the fitting flange. The transmission case is aluminum and spot faced at the boss. The factory figured out that this was a bad design, so they goop it up with threadlocker to prevent loosening. I prefer a copper washer in this application...unless you use thread locker. (which I hate.) The nylon washer is not strong enough to allow you to tighten the fitting to prevent loosening.

And yes, I would really like to see an o-ring interface on that fitting too.