I pulled one head off the mystery motor from my 66 tonight. The right one where the 60 psi compression cylinder was.

So far, nothing big and obvious.

The cylinder walls are super smooth and shiney. The bore at the top is right at 4 in per my measuring. Not even 001 over.

It had an all steel head gasket.

Does this sound like an original?

The low compression cylinder is probably due to a valve issue.
As for the bore being 4" I dunno. To get a real reading you need to measure the top and bottom of the bore.
If the motor has some miles on it the hole will have a taper to it. The top is usually the more worn end of the hole.

Thanks. This is my first Chevy rebuild, and I am not sure what I am looking for. I am trying to determine the year of the engine first.

From reading engine id info on websites, I see that the stampings on the right deck pad are intact, so it may not have been rebuilt. It is, (from my memory of last night..), K 08 10 CMR This should indicate a 74 or 78 350, or an 80 305. Since it has the 4 in bore, I am guessing 350.

The casting number on the rear left by the bellhousing face, 397001 ( from memory) , indicates either a 327 or a 69-79 350.

I have not decoded the heads or casting date stuff yet.

I should have enough info to buy rebuild parts already?

Was a plain steel shim head gasket a stock item?

Steel shim was stock, I think? it was .040" but don't hold me to that. you need to be measuring the stroke, 3 1/4 inch would be 327, 3" would be 302, 3.48 (almost a 1/2) would be 350