Well, contrary to normal replacement practice, re-using the old copper washer has stopped the leak. Whew!

Copper washers can be reused since it usually takes a few uses before they work harden significantly. I always anneal mine with a small propane torch before reuse because I like to play with fire. Annealing is simple, is dimmly lit room heat the copper until it just starts glowing when the heat is removed. It's now annealed. I usually use some fine sandpaper to make the washer shiney, I suppose this also makes sure there is no thick oxide scale on the washer but that has never been a problem.

Kelley

The old washer seems slightly larger in diameter, but not thicker. I'll try it though. Always thought cooper washers were single-use, unless annealed if a new one not available. Can't hurt; might help. Tx

Been awhile since I've had that fitting off.  Is it a symmetrical fitting or does it possibly have a short threaded end and a longer threaded end?  If it's slightly different pehaps it's wrong way round and the longer end is bottoming in the block before it tightens on the copper washer?  Or perhaps the washer is just marginally too thin? Still have the old one to compare it to?  Hell, for that matter if you have the old one try it again...been there, done that in my younger days.

Thought my oil cooler lines were rubber and due for replacement. Turns out the heat shrink tubing the P. O. had put on the Aeroquip lines was what was wrinkled. Sliced that off and re-installed the lines after cleaning everything up.

I replaced the copper washer behind the fitting to the block, and used plumber's tape on the outer thread that the cooler line threads on to.

Seems oil is leaking from where the copper washer is, and I don't understand why. I replaced the washer with a new copper washer, tightened it down, and then tightened the oil cooler line to that. I "think" I tightened the fitting down on the copper washer enough, but wonder if there is a torque spec. It's definitely leaking around the copper washer, not any other fitting. Ideas?