Look what I found when taking off my exhaust manifolds....

[Ryan Stout]

I'd pull the manifold and weld an old 11mm socket to it. Who uses an 11mm anyway?

[bobdec]

Can't really make out in pic if its a front, back or middle bolt. On the LT1 heads the #'s 1,7,2,8 outer positions have two threaded manifold mounting holes in the head about 1/2" apart. Stock exhaust manifolds use the inner hole, headers may use to outer. My Pacesetters used the outer holes. That's helpful if it's an end bolt, and things go south, and your headers use the outer hole.

[nstg8r]

Can't really make out in pic if its a front, back or middle bolt. On the LT1 heads the #'s 1,7,2,8 outer positions have two threaded manifold mounting holes in the head about 1/2" apart. Stock exhaust manifolds use the inner hole, headers may use to outer. My Pacesetters used the outer holes. That's helpful if it's an end bolt, and things go south, and your headers use the outer hole.

It's the passenger side rear most bolt, I'm putting Kooks stepped headers on it.

[Fastbird]

I'd pull the manifold and weld an old 11mm socket to it. Who uses an 11mm anyway?

You'd be surprised. I've reached or it a few times working on things.

OP.....as already suggested, pull the manifold and use vice grips to work the bolt out. There's plenty to work with there.

[nstg8r]

You'd be surprised. I've reached or it a few times working on things.

OP.....as already suggested, pull the manifold and use vice grips to work the bolt out. There's plenty to work with there.

Yeah, I'll try that. Sure is nice and tight under there. Thanks GM engineers!

[Featherburner]

Looks like someone cut the head off and used it as a stud for gasket/manifold alignment and couldn't get it out afterwards.

[Paulster2]

If you are pulling the manifolds off anyway, I'd get them off, hit the bolt with a torch until cherry red, then put your PB blaster on there (or if you have some bees wax, use that). The rapid heating/cooling action along with the penetrant will cause the bolt/head connection to break. Then put your vise-grips on it and it should come right out without any fuss or muss. This should also prevent you from pulling the threads out of your head as well.