[Bill Hanlon]

There is a discussion going on over at the OldGMCtrucks web site that you guys might be able to help with. Many of you know that GMC used Pontiac V8 engines as an option in their smaller trucks from early 1955 through the end of the 1959 production run. The 55, 56 and 57 GMC engines were the same cubic inch displacement as same year Pontiac engines. The '58 GMC engine had a smaller bore at 3.875", yielding 336.1 cubic inches instead of '58 Pontiac's 370 cubic inches.

Finding pistons for the GMC engine has become very hard to do.

My question is "Did the Pontiac and GMC engines use the same casting number?" If they did, one should be able to bore the GMC block to standard 370 piston size (or 347 piston size if you wanted to be more conservative).

Any of you that have a '58 370 willing to get the casing number from your Pontiac?

[Stuart]

Per Pete McCarthy's book, 1958 370ci blocks had a casting number of 528456.

[Bill Hanlon]

Thanks!

[Teej]

I've got a 58 in my car and what I believe to be a 57 on the floor of my garage. I'll check the casting numbers tomorrow and post what I find out.

[Teej]

The dip stick is in the way of the casting number on my '58, but what I can see looks like 528?56 and then it has a GM3 cast toward the front. So that jives with what Stuart posted.

The '57 GMC engine I have on the floor has 523283 and a GM7 toward the front.
The heads are #523298 and the exhaust manifold also starts with 523...

[Stuart]

Originally Posted by Teej

The dip stick is in the way of the casting number on my '58, but what I can see looks like 528?56 and then it has a GM3 cast toward the front. So that jives with what Stuart posted.

The '57 GMC engine I have on the floor has 523283 and a GM7 toward the front.
The heads are #523298 and the exhaust manifold also starts with 523...

Pete's book says 1957 Pontiac engines have a 523293 casting number, can you double check your block? Sometimes 8s and 9s look the same on those casting numbers. The head casting number agrees with his book for 1957.

[Teej]

From what I understand, this is a GMC block.

[Stuart]

Yup, that's an 8. That could be the difference between a PMD and GMC block, or the book could have a typo - my copy is pretty old, I think he updated the book and made some corrections at some point.

[johnta1]

Does that block look the same as a Pontiac one?

Same mounts or bolt hole locations etc?

Interesting.

[Teej]

I believe it has the same mounts but my only '58 for comparison is bolted in my car and so I have limited access. It's got the single mount under the timing cover up front and then the two that were on the bellhousing, and they look like my '58.

I can take some measurements or more photos if anyone wants them. Just let me know what to get.

[Bill Hanlon]

Teej:

I think a GMC block will have the engine serial stamped on the passenger's side of the block, just above the oil pan mounting rail and just forward of the drain cock on a machined surface.

A Pontiac block would have identification numbers stamped on the passenger's side front of the block, just below the head.

Which does this engine have?

- Bill

[Teej]

GMC Location 0158214

[Bill Hanlon]

Originally Posted by Teej

GMC Location 0158214

I don't understand. If in the GMC location, the number should have been something like 336012345 for a '58 block.

[Teej]

Got me...

I know my '58 Pontiac is stamped just below the head. Don't have the number off hand, but the last digits match my VIN.

I can take a picture of the numbers, but they are pretty easy to see. It is possible I missed the first 336 or something, but I don't think so.