I took the intake/exhaust pair off the spare engine, preparatory to tearing it apart and replacing at least the rings. In doing so, when I removed the rearmost nut for the intake upper stud, the part of the intake manifold through which the stud passes fell off, along with a fairly good-sized chunk of the runner. I've no idea how long it's been broken.

Has anyone ever welded one of these? I'm a pretty good TIG welder and have a mill so I could reface it afterwards and get it flat, but I don't know if the aluminum is a particularly poor alloy for welding.

Happens often enough. Seen a few welded ones which broke as ypu described. Generally pretty rough but most would have been done prior to tig welders being commonly used. I reckon it should be an easy job for you

Cool, thanks. I'll give it a shot. It probably got overtightened at some point.