Billet Tube Oil Cooler Coolant Leak

[QuantA8]

Hello everyone, new to the forum here so I apologize if this is a question which has been asked. I did a search and found nothing on my specific issue, though plenty of recommendations to get the aluminum piece I have installed in the car.

I have a pre-facelift A8 that I recently did the timing belt/timing parts, cam/sump/crank seals, the front plate seal, and the oil cooler tube (which was replaced with a billet aluminum one). The leak appears to be coming from the new billet tube which connects the oil cooler to the block, as it is directly below that place and the only other option for coolant from right there is the head gasket or the rear coolant hose (which is also on the oil cooler). I am 99% sure it is not the rear coolant hose because that one had a small initial leak which I fixed by putting stopleak in the coolant system (or which went away the first time the car got warm :/ ), and the leak for that comes down off of the oil coolers coolant drain plug, whereas this leak is right around the alternator bracket, just a few inches closer to the front of the car. I have no reason to believe it is a head gasket failure other than it is a possibility for why I would see the coolant leaking.

The leak is a very slow leak that only happens when the engine is cold (so the stopleak is no help). I don't think it is a head gasket that just so happens to be leaking directly above an obvious coolant port and the coolant is just evaporating off the block when the engine gets warmed up, as I don't see any drips until the engine is below 100F or so on its exterior surfaces. So it only happens when the engine is totally cooled off, and it comes from directly below that coolant tube. As a result it appears to me to be a situation where the cool tolerances of the motor/coolant tube make it such that when the motor is cool I get a small leak there, though this seems strange given I had to use the bolts of the oil cooler to force the oil cooler all the way over the coolant tube since it fit that hole so snugly I could not easily shove it on all the way by hand. The tube also has two o-rings on each end, so if the seals failed at least two seals failed, so again I have a hard time seeing how that would happen.

As it is, it just leaves a small 2-3" disk of coolant anywhere it sits overnight, so it is annoying enough to me to be worth the 7-8 hours it would take me to replace the part since the car is still my daily for a while longer. My question is if people think my diagnosis is good, and if so what the best fix would be. I am hoping this is some kind of known issue (cracked/deformed pipe? failed seals?) and I just got the wrong aluminum part, and that the right part would do the trick. There is always the original part, which is very unappealing because it is a known failure point and it was cracked and leaking on my car and was the straw that inspired me do the work I did. Thanks for any help :)

[QuantA8]

So I gave it a 1% chance. In retrospect that was unfairly low...

I finally got directly under the leak yesterday and to my surprise I could clearly see that the leak was coming directly down toward the drain plug as it would were it to be the rear coolant line, but then it veers off toward the front of the motor and drips off the alternator bracket which is directly below the coolant port that connects the oil cooler to the block. So the initial leak from the rear coolant line went away or was just unnoticed for a few months, then came back but from a different place and in just the right way to mimic a different leak. In any case, nice to have that solved, and lol.

On that note, anyone have any advice for how to get that line to seal? It's just a metal coolant line that is pressed into the oil cooler and the line has an o-ring on the end of it, so its not complicated in any way. It has a limited range (close to 1mm at most) of where it can be due to the oil cooler being rigidly bolted to the motor and the line being rigidly bolted to the engine. I was surprised it leaked initially since it seemed like the simplest part of the whole job, and I am surprised just getting the engine warm didn't set it in place. As a result of my inexperience and the simplicity of it, I am unsure of what the cause likely is since it is a metal tube in a metal hole and it is bolted in place. The o-ring was replaced and a bit of the same sealant used to seal the water pump and thermostat were used around the o-ring. Any help is appreciated.