Here is a recent radiator experience update. 1994 D1, 4 year old plastic tanks, 2-row aluminum fins type radiator. Developed a slow leak at the outlet (lower) flange. It is a two bolt flange with an o-ring seal. It was leaking even with pressure cap backed off a turn (thanks to the Forum recommendations). I noticed a couple weeks ago, but had a very hot run up to 220 deg F in line for fast food this summer, and have been adding half a quart of coolant regularly since then. Turns out the metal insert nuts have backed out a tad, pressing into the plastic flange. That is the root cause.

It had a drip out of the box there (with full pressure), so tightened a fraction of a turn and it stopped for about three years. This time, I removed the flange (still in truck, the bolts seemed loose), replaced the lousy stock o-ring with a good one, and used an RTV skim coat. No leaks so far. If it leaks again, my next step is either a stop leak or a new radiator, depending on my mood.

The stock o-ring did not look straight. So, I recommend taking a look at it if you buy one. If there is a next time, I will disassemble that joint and apply some RTV or replace the o-ring including an RTV skim coat. Of course, a metal radiator won't leak there, so that is an option too.