i need to use a manifold on my vortec heads that uses the outermost bolt hole drilled on the older SBC heads. vortecs don't have this hole for some reason or another.

i figured i would be slick as all hell and make my own flat area, 3/16" recessed from the exhaust flange on the head because it was the shallowest i could cut and get a flat surface, and then drill/tap it for my manifold bolt. i whipped out my tools and cut myself a nice straight flat surface to drill, put the drill to it, and didn't even need to press. went straight into water jacket. bummer. my buddy thought i ruined a good set of gmpp heads.

luckily, i slept on it and busted out the tap set and drill index. i drilled until there was enough meat around the outsides of my hole to take a pipe thread, which ended up being 1/4" pipe thread. then i put a plug in the hole. so its fixed and fine.

however, as a public service, i must say don't try that with vortec heads or you'll be boned too.

on another note, i wound up engineering and building a pretty freaking ingenius way of clamping/bolting on the end hole of my manifold so as not to have a gasket blowout. pics will have to follow

Oh yeah, that funny "double" exhaust bolt hole on the end of some SBC heads. I know what you're talking about. That is definitely a bad way to find that out. Glad to hear you got it fixed, though.

What uses that bolt hole anyway? I can't recall anything that used it off the top of my head. Certain exhaust manifolds? An accessory bracket?

passenger side 3rd gen f-body used it - thats the manifold i'm using(on my drivers side). its the only thing i've ever seen use that hole as well.

Alot of the 305's used that odd placed bolt hole...

patgizz.. Sorry to hear about your bad luck there, but glad you got it squared away

Truck manifolds.....

And they don't line up too well on 74 engines too well.....well, they WILL line up but you'll be missing the last bolt hole....

Don't ask my WHY they did that....

3rd gen manifolds? I have a set in my garage right now. I'm gonna have to take a look at the bolt pattern on them and see if they use this weirdo bolt placement, too.

Yeah, I've got manifolds from a 3rd gen 305 laying around as well - funny with that one bolt hole being further back.
I think they did it to allow for enough room inside the manifold because the manifold dumps in a different place than the older style manifolds.

Pioneer makes an adapter plate to use a new style exhaust manifold on an old style head.

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i had 0 room for an adapter of any kind... 1/2" clearance to my steering box with gasket installed, solid mounts so it won't move.

like night rider said its for 305s there manfolds are diffrent just one side used that hole

At least it was water this time Pat and not fuel http://www.nastyz28.com/ubb/wink.gif

I just checked a set of 1987 L-98 350 TPI manifolds I have laying around..... yep, they got that funny bolt hole location at the very back of the pass. side manifold. So it's not just 305 manifolds, it's like that for (all?) 3rd gen V8 exhaust manifolds, I guess.

I think they did it becuase the downpipe goes right down at a ~45* angle exactly where the rear-most exhaust port is located in the head. In other words the whole exhaust log has to turn down and squeeze between the 2 rear-most exhaust manifold bolts. That would be a pretty tight (restrictive) squeeze if the didn't spread those 2 rear bolts apart some.

A 92 Caprice (and the other years that use a non LT-1 engine) uses that funky bolt hole too, for the same reason as Damon pointed out.