Help ID this 538181 Block
 

Bought a 61 Bonneville, CA built car. Was told it was likely 389, thought it was a 389 until I looked at it.

538181 Block and has the 421 transfer lug, 538177 Heads. Had to PB/brush a lot of crap off the date code, best I can tell with a borescope it's C 28 8 (look like 8's which would be incorrect) or C 2 3/8 3. No visible stamp on the metal under the passenger side head even after using acid to clean/etch. I may have to pull the timing cover to find something? I do not see a chamfer at the top of the cylinder with the scope. I still have to scope to see if it's 4 bolt.

Thoughts on the date code and no markings on the front pad?
I'll upload photos from the scope once I find the card for it.

FYI,all non 389-400 pass car engine did not have the transfer lug.Tom

All blocks for 1961 and 1962 including SD's had a 538181 casting number.
All non-SD heads for 1961 and 1962 had a 538177 casting number.
Are you sure there's no numbers at all on the front of the block? For those years there should be both an engine code and the VIN, they could be covered up by paint, rust or grease/dirt. I've never seen one without them, could it possibly be a service replacement block?

Here's a picture before I used the razor blade, the scratch marks are courtesy of the previous owner.

I was thinking it could be a service replacement too but have no supporting information. I can't find any record of this casting number being made into early 63. The 65 421 I have has a very illegible date code too, maybe I'm just seeing a 3/8 and it's really a 1 or 2.

The motor runs well enough that I want to keep it together, but it's a mystery that's eating at me.

That is weird. It may be possible that numbers are hiding under that little bit of rust if they were faintly stamped, but it doesn't look like it.

block numbers
 

I bought a 63 421 short block over the Pontiac parts counter years ago to use as a basis for a SD engine. I have that engine in my shop and it does not, nor has it ever had any numbers stamped on it. FWIW. Mike

Here are the scope photos of the date code and a clean front pad.

421Mike, thanks for your reply. I think it may be safe to assume it was a replacement.

Here’s a pic of the exact location of the front numbers on my 62 GP.

I dont think OTC blocks or engines were stamped with code back then.If not destined for a car not needed?Tom