How I check a manifold for casting flaws before install - steps that saved me a return trip

I took the approach you described when replacing the manifold on my older 3.8L engine last week. I had a few of the bolts come loose early in the process, so I was nervous about whether the casting was solid before tightening them down. Using that stiff piece of wire, I poked at all the exhaust ports, and on the one problematic manifold, it stopped dead. It was clearly just a surface indentation from the casting process, nothing that looked like a through hole. I also gave the flange edges a good visual inspection under the shop light, and it looked okay despite a little grime. I decided to clean out the areas with the wire brush and proceed with the installation. Once it was fully torqued, I drove it for a few days and checked under the hood for any signs of blue smoke or odd leaks around the mating surface. Everything stayed quiet, so I guess the surface pit wasn't an issue for this part. It was definitely cheaper to inspect it than replace the whole assembly again.

I'm still trying to figure out the best way to handle this. The wire test seemed pretty definitive, but I've been looking at my own casting and worrying about the internal stress near the runners. I wasn't sure what to look for besides a clear hole.

I happened to use a straightedge to check the flange area on the part I got, and there was some very slight warping on one side. I wonder if that kind of distortion is just as bad as a pinhole, even if it doesn't go through completley. It makes me nervous to put it on because of the heat cycles.

It is understandable why the internal structure near the runners causes concern, especally given how rough those factory castings can be from the initial mold. Once you have ruled out an obvious through-hole or major cracking, the real long-term risk shifts to thermal cycling and subsequent metal fatigue. The expansion and contraction caused by the heat load during a drive can exacerbate any microscopic structural weaknesses even if the surface appears sound. Have you considered the possibility of insufficient torque being applied across all the flange bolts? Proper clamping pressure is critical for maintaining the seal and distributing the load evenly across the mating surface.

I get the hesitation, but my expirience leaning towards caution. I once tried sealing a similar pinhole on an old header, but the fix only held for a month before the heat started vibrating the seal apart and causing a bigger leak. I decided that wasn't worth the hassle or the cost of the ongoing exhaust fixes. For me, the biggest deciding factor was the material I used to repair it, and even that didn't solve the underlying issue over time. Sometimes it's better just to bite the bullet and get a proper unit, even if the initial defect seems minor.