Strange 3973487X casting date--need help!

[Mike E.]

I'm cleaning the engine on a 72 base-motor turbo 400 car that has 60k miles, and is pretty much untouched. Alternator has been replaced. In take the valve covers off, and cleaning the area between the intake and the covers, there appear to be original tabs on the intake gasket. However, the right head has an M 114 casting date. There is a significant space between the M and the 114. Several questions: a) when did they ever use an "M" in a casting date month at that time? I thought they used an I in casting and not in stamping, which makes an L the 12th month. b) was the 3973487X head cast in 1974? c) The other head is an A52 head, so an L head on the other side would be appropriately dated.
There's no indication on any of the intake bolts or exhaust bolts that they have ever had a wrench on them--shielding brackets are still in place, etc.
I'd appreciate your thoughts.
Thanks!
Mike

[David L.]

Re: Strange 3973487X casting date--need help!

Mike,

According to "1970-1975 Chevrolet by the Numbers" (page 98) The 3973487X "cylinder head is identical to the 3973487 except that it was only used in late 1972 model year cars. It is unclear why there seems yo be an overlap in usage with # 3998993. #3998993 was released about January 4, 1972."

Your "A52" cyl. head was made one day after this release.

It appears to be a mistake in the casting date during this change.

Dave

[David L.]

Re: Strange 3973487X casting date--need help!

Mike,

In the last year or so I find myself looking thru old issues of my 30+ year collection of "The Corvette Restorer". This morning I coincidentally came across an article entitled "1972 CORVETTE SPECIFICATIONS" compiled by John Amgwert (Vol. 12, #1, Summer 1985, page 29). The part about cylinder heads is as follow:

"(X) indicates that an "X" casting foundry proofmark will appear after the seven digit casting number on later 1972 base and LT-1 39734487 cylinder heads. An "X" proofmark is used by Central Foundry Division with any new casting or casting composition to alert the machine shop and warranty system that casting failure is to be reported immediately."

Dave

[Cecil L.]

Re: Strange 3973487X casting date--need help!

Mike,
I posted a copy of a letter from GM on this subject but can't re-post it. It can be found here:
https://www.forums.ncrs.org/showthread.php?t=77428

Added here is a copy of the letter prompting the response.

[Kenneth H.]

Re: Strange 3973487X casting date--need help!

And here's the attachment to the above letter.