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I have a stock radiator for an F-Body with an LT1 (around 1995 or so). This is a new radiator. I had no problem with the top trans line fitting, but the bottom fitting won't stop leaking. I have tried it with teflon tape and without (I have yet to try paste). It continues to have a leak and when I tighten it a little more it strips.
The leak is coming from where the male end of the fitting below goes into the cooler, NOT from the 3/8" transmission line side. So, the using the picture below, the leak is on the male end of the treads.
I have thoroughly cleaned all of the threads from the last fittings out and my next attempt will be to try to use paste, since I've tried everything else. The threads inside the female portion appear to be undamaged.
It is flared right? For flared fittings the threads do not seal, the flare itself is the sealing surface. If the threads are leaking something fishy is going on with the flare
That's what I thought as well, but these appear to be straight cut threads and not tapered. I'm not sure, but even according to some of the parts store websites this is the correct fitting. I have tried to locate the degree of the flare but cannot find it, but it appears to be right, and the top fitting hasn't been any trouble at all.
Flare refers to an angled convex face meeting and pressing against an angled concave face. Tape or paste at the threads will do nothing. AN fittings are a textbook example, but flared transmission lines (as your Caprice came with in the '70s) are another example.
Threads should indeed be straight per the spec. sheet. Now, ignore the threads and look at the mating faces within the cooler, and at the cooler side of the replaceable fitting. Metal junk, an uneven or scarred face or other reason the faces are not smooth and able to meet flat against each other are probably causing the leak. Compare the bad pieces to the good one that's not leaking.
Flare refers to an angled convex face meeting and pressing against an angled concave face. Tape or paste at the threads will do nothing. AN fittings are a textbook example, but flared transmission lines (as your Caprice came with in the '70s) are another example.
Threads should indeed be straight per the spec. sheet. Now, ignore the threads and look at the mating faces within the cooler, and at the cooler side of the replaceable fitting. Metal junk, an uneven or scarred face or other reason the faces are not smooth and able to meet flat against each other are probably causing the leak. Compare the bad pieces to the good one that's not leaking.
Good info here. Try swapping the adapter fitting from top to bottom and see if the leak moves with it to try to eliminate one of the variables.
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Flare size for trans line adapter fitting 
