Need Help With a Pontiac Question.

Hi guys. I'm in a disagreement with a guy at work over the 400 cid engine used in the GTO. I am under the understanding that this engine was a big block. He's claiming that it is a small block. I know that Chevy has a small block 400, but I have never heard of a Pontiac small block 400. Am I mistaken here? Thanks......John.

 

You're both wrong. No big or small block designation for the Pontiac V8s. Really.

Someone more in the know can give the technical explanation but that's the skinny.

 

Pontiac blocks from 1964 on up are the same from 326 - 455. Pre 1964 stuff is similiar, but a little different. That's what makes them fun sleepers. Tony

 

The Pontiac 287 and 316 had 2.50" mains.
The 347 and 370 had 2.625" mains.
The 326, 350, 389, and 400 had 3.00" mains.
The 421, 428 and 455 had 3.25" mains.

Larry

 

For those of us who are still learning (like me) how do you define small blocks and big blocks?

 

With Pontiac they literally only have 1 block, so no point in classifying them as either. Most manufactures have a smaller displacement block, and a different design for larger displacement. I don't think their is a specific number that makes it either, just which design it is. A 400 Chevy is a small block, but they have a 396 and 402 version of the big block. So the waters get kinda muddy.

 

I don't think that's right, 389 and 421 parts are interchangable (my brother has a 66 pontiac 389)

 

[QUOTE='60buickrod]I don't think that's right, 389 and 421 parts are interchangable (my brother has a 66 pontiac 389)[/QUOTE]

The factory shop manual lists the 389 @ 3.00" mains and the 421 @ 3.25" mains. My old catalogs & books from H-O Racing concur. I never had a 421" apart, but my old '63 Poncho w/389 had 3.00" mains. Since the cylinder spacing is also the same, I suppose it's theoretically possible to cut the 421 mains down to 3.00" and drop it in a 350, 389 or a 400. In that respect they would be interchangeable.

But although my response was vague, my point was that except for main bearing diameter, the Pontiac blocks were essentially the same, and as long as certain precautions are taken, many parts like heads, intake and exhaust manifolds, cams are interchangeable, which is not the case with Chevy, Chrysler, Buick, & Ford.

Cheers.

Larry

 

Just out of curiousity is the late 70's Pontiac 400 the same as the one made during the sixties?

Patrick

 

Basically the same, just different heads, compression, and casting numbers.