1969 L48 "M" ystery casting - NCRS Discussion Boards

1969 L48 "M" ystery casting

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  • Dan Pepper

    1969 L48 "M" ystery casting

    Yesterday I trailered a '69 350/350 Monaco Orange coupe with 79k original miles from San Jose home to Seattle. The car is October '69 build #32444, sold new by Harry Mann Chevy in LA (so says POP). Practically every mile, tank of fuel and service has been recorded in 2 previous owners "little black books", so I'm pretty sure it's mostly original (except starter, H2O pump, battery, smog pump, etc.). Upon checking the engine codes, I discovered that rather than a casting date, the car is coded "M47" where the date code should be. I read an article in the restorer by Gene Gamache, that explains the "M" code blocks were "specials" the factory used for experimenting on ways to improve the production process.

    In addition to the odd date casting, the car also has a 1-12-9 date on the intake. Also, the left cylinder head is casting 3921786, dated 1-19-9 (read the #'s thru the oil filler hole). This number falls in line with the intake casting - so based on the cars documetation I'm guessing the right head is dated close to the left. Both intake and head(s) have a January date, obviously very late for the Oct. build date.

    Could this engine have been a "test" engine early in the year and kept off to the side until the end of the production year when it was installed? Can anyone shed any more light on the "M" coded engines? Or why such an early intake & head(s) might have been used?
  • Joe L.
    Beyond Control Poster
    • February 1, 1988
    • 43193

    #2
    Re: 1969 L48 "M" ystery casting

    Dan-----

    What is the casting NUMBER of the block?
    In Appreciation of John Hinckley

    Comment

    • Dan Pepper

      #3
      Cylinder Case casting # 3970010 *NM*

      Comment

      • Mark G.
        Very Frequent User
        • March 1, 2001
        • 227

        #4

        Comment

        • Dan Pepper

          #5
          It's "I" (for September) not "1"

          Ooops. Just figured out that the "1" on the intake and head casting dates are "I"'s - and represent September '69 casting dates. These would be correct for an October 4th built engine.

          The question RE other '69 "M" casings still stands. Anyone else out there have a '69 "M" date coded 3970010 casing?

          Comment

          • Terry M.
            Beyond Control Poster
            • September 30, 1980
            • 15573

            #6
            Re: 1969 L48 "M" ystery casting

            Dan,

            The above referenced story lists one other 1969 M-coded small block, but it is a 3955618 casting. Further, according to that story there are eleven 3970010 cylinder cases with the M code listed between 1970 and 1972 production. I also have an unconfirmed report of one on a 3970010 cylinder case in the 1974 or 1975 range as well.

            It is ironic that the title of that story is almost identical to your subject line. I would be happy to receive the data from your car for my files. I expect Gene Gamache would be interested as well.
            Terry

            Comment

            • Dan Pepper

              #7
              Re: 1969 L48 "M" ystery casting

              Terry-

              As mentioned in the first post, I have read Gene's article on the "M" casting - hence the subject line. I'm interested in learning of other '69's with this code. I'll be happy to send you more info on the car.

              Comment

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