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First, I am new to the Pontiac World.
I picked up a 350 from a 1970 LeMans. It has #11 heads on it. They are D-port but they only have 4 bolt holes on the exhaust flange. 2 in the center and then 1 on each of the inner parts for the outer flanges. Nothing on the 'ends' of the heads.

What prevents the exhaust manifolds from leaking?
 

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The exhaust manifold flange is thick and does not bend easily. Pontiac omitted the end holes on lots of heads. Most of the '72 heads don't even have a flat surface on the end, where a hole can be drilled. But, I think those #11 heads do have a flat surface which can be drilled & tapped, if you wanna go to that trouble & expense.

Some even drill the '72 heads & use a long stud. Also there are angle brackets which can be used on the '72 heads.

Hey, welcome to the "Pontiac World" ! And good luck with your Pontiac. :)
 

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In an effort to save money, Pontiac first eliminated the hole & stud, but left the machined pad/casting in place. They they decided to save a little more money on the casting and cast the heads without the end material to machine a pad.

Their thinking was that the cast iron exhaust manifold was rigid enough to seal on the ports/gaskets. So, the heads work with cast iron exhaust manifolds BUT the problem comes into play when you want to use the factory cast iron Ram Air exhaust manifolds or aftermarket headers which have much thinner steel flanges. Use headers and they will leak, and I suspect the RA manifolds would as well.

bigD provided a great pic of the heads with the machined pad. It can be drilled and tapped by a competent machine shop and an exhaust stud added.

The other style heads do not have that nice pad to work with. I have read that some will drill and tap an exhaust bolt into the corners, but they will break into the water jackets so they needed to be sealed and there is not a lot of material there. Does not sound like a good option in my book.

The other option are a set of "L-brackets" which bolt to the head and provide a stud that allows you to use the end holes on the factory style cast iron Ram Air exhaust manifolds or aftermarket headers to tighten them up. Here is a description and price from one vendor: https://www.opgi.com/gto/G240117/
 

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