I've never been able to get this manifold/Y-pipe connection air tight (engine stone cold). Should it be or is this just a quirk of discos?

 

Can be. I have found success with these gaskets https://a.co/d/0QtrHFM and of course stainless studs and copper nuts here: https://extinctmotorsports.com/exhaust-studs

 

Aw man, I had the opposite experience haha. I put those in and after about a year I was driving home one day and was like... jeez, the Disco sounds like one of those brodozer trucks. Crazy loud. Popped the hood when I got home to find one side completely missing a gasket, and the other was barely holding together. I took off the Y pipe and dumped out a ton of Remflex nuggets that the cats had caught.

I torqued them following the package instructions but who knows. Maybe I got a bad batch ¯\_(ツ)_/¯

 

I've never been bothered by whatever minute exhaust leaks may or may not be happening. I always get condensation drips on the driveway below where the Y pipe meets the rear section on cold start ups so I know it's not sealed, but I never really heard anything out of the ordinary. I just use the stock gaskets at either side of the manifolds and crank them down as best I can. Maybe they're leaking??? It's not offensively loud, or even noticeable for that matter, so I just go with it. I put new gaskets on the manifold to Y pipe connection the other day and immediately got a CEL on the driver's upstream O2 so I took it back down, redid the O2s over a couple days and put it back up wit the used the mani gaskets. Seems fine. I haven't don't the pressure test, but I'm not sure I want to. I'm half expecting it to leak. No problems at inspection and nothing audible so to me that means "no noise, ...no problem".

 

There is a solution but it bloody time consuming, pull the y-pipe and manifolds. Then pull the studs and deck both mating surfaces to perfectly flat, and reassemble leaving the rear connection of the y-pipe until last. Finally attach the the rest of the exhaust.
Personally I just replace the gaskets when the noise annoys me.

 

The y pipe connection on my d2 gave me so much grief over the years. Make sure your tcase/trans mounts and motors mounts are in good shape because if any one of them are sheared, it'll eventually lead to a stress fracture on those flanges. I really think there should be some sort of flex pipe on those sections to aid in engine movement. If the headers weren't cast manifolds, I'd probably cut the flange to do a vband or something on them because they're so annoying. A leak there leads to all sort of fueling weirdness and can affect smog in CA since its before the primary o2 sensors.

 

The exhaust pipe that connects to the manifold needs to be loose at the other end, for it to press together properly. An exhaust leak can be very costly because it’ll destroy your o2 sensors and then your cats, pretty quickly.

Those gaskets can’t compensate for a misaligned connection if the two mating ends aren’t perfectly parallel.