427 Closed & Open Chamber Pistons - NCRS Discussion Boards

427 Closed & Open Chamber Pistons

Collapse
X
 
  • Time
  • Show
Clear All
new posts
  • Steve Antonucci

    427 Closed & Open Chamber Pistons

    Hi everyone,

    Recently, I posted a topic here about the differences between all of the BBC
    dimple rods. It was great to get so much feedback and information. I now
    have a much better understanding about them. My thanks to all. This forum is
    truly the best venue for this type of research.

    As some of you may recall, the BBC dimple rod discussions got into early vs.
    late and/or 1st vs. 2nd designs for these rods and at some point, the topic
    migrated into closed vs. open chamber designs.

    To that end, I would like to ask some questions relating to the 427 engine and
    the use of closed vs. open chamber pistons.

    When exactly did the open chamber design come into production? Was it for the
    late 1969 L-88's and all ZL-1's? Was it first in aluminum BBC heads, then
    cast iron later? Then, there is the really odd 3933148 cast iron head that
    used an almost full open chamber design. This was used in the 1969 model year
    on the 396-265hp ( 2-bbl Rochester ) full size cars.

    Now, it gets a little more involved. My '69 L/89 used the 3919842 closed
    chamber aluminum heads. Over the years, I have seen a few '842 heads that
    look more open chamber-like than closed. My '842 heads have a combustion
    chamber that is the standard bathtub shape. I have seen '842's with the
    spark plug side layed-back at an angle more like the open chambers.

    Would not that change require a different casting or part number? Or, could
    these '842 heads have been owner modified to reduce compression sometime in
    the past? Can this modification be performed on aluminum heads safely? Due
    to core shift in the casting process, I remember reading that this type of
    combustion chamber modification was risky. Do any members here remember
    any '842 heads with these differences?

    Lastly, I know that closed chamber pistons can be used with open chamber heads,
    but the inverse does not apply without dire consequences. If I am trying to
    build an L-88, what piston should I be looking for to match up with the '842
    heads, assuming I was to start with stock items from 1969? Were the 3946074
    aluminum heads used on late introduction L-88's? Or even early L-88's? What
    piston would match with these heads? BTW, did the L-88's use the '842 heads
    in 1969? What about '67 & '68?

    I know there were probably a lot of BBC combinations from 1967-1972 as they
    relate to this topic. I hope some of you can help me understand how this all
    fit together.

    Yes, another BBC history lesson in the making.

    Thanks,

    Steve
  • Joe L.
    Beyond Control Poster
    • February 1, 1988
    • 43193

    #2
    Re: 427 Closed & Open Chamber Pistons

    Steve-----

    Very early 1969 L-88 probably used the GM #3919842 closed chamber aluminum heads. These heads were also used for all 1968-69 with L-71/L-89. All GM #3919842 heads originally had the same combustion chamber. For the most part with GM heads, combustion chamber configuration is part of the casting. GM was not doing CNC machining in those days.

    Most 1969 L-88 and all 1969 ZL-1 used heads GM #3946074. These were the open chamber aluminum heads. Just when the change occurred, I do not know. I would expect fairly early in the model year.

    The GM #3933148 cast iron cylinder heads were the first semi-open combustion chamber heads released by Chevrolet. They were used for the applications you describe. This semi-open chamber design head was never used for any HP or SHP applications. (However, MUCH, MUCH later and for SERVICE-only, a DIFFERENT semi-open combustion chamber design was released. This design is better than full-open design for performance purposes).

    The first cast iron open chamber head was the GM casting #3993820. This head was used for all 1971 big block applications except Corvette LS-6. It is a 2.06/1.72 valve size head with the large oval ports.

    The only open chamber, cast iron, rectangular port, 2.19/1.88 cylinder head ever intended for PRODUCTION use was the GM #3994026. This head was intended for 1971 Chevelle LS-6 applications, but no so-equipped Chevelles were ever released. This head was available in SERVICE, though. Later, it became the GM PART #6260482 (casting 6272990) cylinder head which was also a SERVICE-only head.

    The standard pistons for the 1969 L-88 with 3919842 heads was GM #3909857. These pistons are designed for use with press-fit rods. The standard pistons for 1969 L-88/ZL-1 with the 3946074 heads were GM #3959105 which are designed for the floating pin rods. Both are long-since discontinued and probably near-impossible to find.
    In Appreciation of John Hinckley

    Comment

    • Steve Antonucci

      #3
      Re: 427 Closed & Open Chamber Pistons

      Joe,

      So what would explain the existance of the '842 heads with a somewhat open
      combustion chamber design? GM experiments in the field? I need to find some images to post for review.

      Lastly, if such a set of original pistons can be located, would you consider
      using them? I'm not holding out for NOS pistons here Joe. THAT, would be
      impossible.

      Steve

      Comment

      • Steve Antonucci

        #4
        Re: 427 Closed & Open Chamber Pistons

        Joe,

        Does your wealth of reference material show the application for this
        piston: 3947886

        It is reported to be a 12.5:1 compression rating.

        Steve

        Comment

        • Clem Z.
          Expired
          • January 1, 2006
          • 9427

          #5
          Re: 427 Closed & Open Chamber Pistons

          open chamber 12.5 piston,similar to 3959105

          Comment

          • Steve Antonucci

            #6
            Re: 427 Closed & Open Chamber Pistons

            Clem,

            Are these service pistons? They don't match the numbers Joe mentioned?

            Steve

            Comment

            • Clem Z.
              Expired
              • January 1, 2006
              • 9427

              #7
              Re: 427 Closed & Open Chamber Pistons

              i think they came in L-88 short blocks or complete L-88 engines when we bought them back then.

              Comment

              • Joe L.
                Beyond Control Poster
                • February 1, 1988
                • 43193

                #8
                Re: 427 Closed & Open Chamber Pistons

                Steve-----

                I can find no record of any finished piston of GM #3947886. As I vaguely recall, that may be the forging number for the GM #3959105 piston. So, I think we are talking about one-and-the-same thing.
                In Appreciation of John Hinckley

                Comment

                • Joe L.
                  Beyond Control Poster
                  • February 1, 1988
                  • 43193

                  #9
                  Re: 427 Closed & Open Chamber Pistons

                  Steve-----

                  I don't know what their thinking was in designing and releasing these heads. It may have been an early attempt at achieving the benefits of BOTH the open chamber and closed chamber design. It's success or lack thereof might be indicated by the fact that I don't think this design was continued.

                  Would I use the 1969 L-88/ZL-1 pistons, NOS or good used? No.
                  In Appreciation of John Hinckley

                  Comment

                  • Steve Antonucci

                    #10
                    Re: 427 Closed & Open Chamber Pistons

                    Joe,

                    The number I asked you about was inked on the top of the pistons in question.
                    Does that help?

                    Steve

                    Comment

                    • Joe L.
                      Beyond Control Poster
                      • February 1, 1988
                      • 43193

                      #11
                      Re: 427 Closed & Open Chamber Pistons

                      Steve-----

                      That's usually a finished part number. I can't find any record of it, though. It might be a PRODUCTION-only part number.
                      In Appreciation of John Hinckley

                      Comment

                      • Mark #28455

                        #12
                        Re: 427 Closed & Open Chamber Pistons

                        The 842 heads were only cast with a closed chamber. Any that you see with an open chamber were a hot rod modification (the open chamber heads did not shroud the valves as much and breathed better).

                        Probably more 1969 L88's were built with the 842 than the 074 heads as the change over occurred after the strike ended. That was also the change over for the 4054 vs. 4296 carb.

                        Some original vintage closed chamber pistons will not work with the open chamber heads. I did not believe this at first, but back about 1980 a friend who owned a speed shop was showing me an engine he was assembling with vintage pistons and the closed chamber pistons hit the open chamber edge - I think it was near the exhaust valve (the open chamber was actually a little tighter to the valve than the closed chamber). I do not remember if this was with the original GM L88 forging with the rounded skirts or the later TRW part with the sharp angle at the skirts.

                        For at least the past 15 years, the dome shape was changed to allow the use of closed chamber pistons with open chamber heads to lower the compression, so if you buy any modern piston, it will likely work OK.

                        Mark

                        Comment

                        • Joe L.
                          Beyond Control Poster
                          • February 1, 1988
                          • 43193

                          #13
                          Re: 427 Closed & Open Chamber Pistons

                          Mark-----

                          I don't think that any of these pistons were ever a GM forging. As far as I am aware, GM never had any in-house aluminum forging capability, at all. They once had steel forging capability at the Tonawanda and Detroit forging operations, but never aluminum.
                          In Appreciation of John Hinckley

                          Comment

                          • Mark #28455

                            #14
                            Oops, I misspoke.

                            Sorry, I misspoke. I was told the original L88 pistons were forged and machined by TRW (but sold as GM parts). The first versions were available through about 1970 or so and were rounded where the skirts came around into the pin boss. The second versions had a sharp angle where the skirt and pin boss met.

                            Mark

                            Comment

                            • Steve Antonucci

                              #15
                              Re: 427 Closed & Open Chamber Pistons

                              Here is an '074 aluminum head that shows a somewhat open combustion
                              chamber design.




                              Attached Files

                              Comment

                              Working...

                              Debug Information

                              Searching...Please wait.
                              An unexpected error was returned: 'Your submission could not be processed because you have logged in since the previous page was loaded.

                              Please push the back button and reload the previous window.'
                              An unexpected error was returned: 'Your submission could not be processed because the token has expired.

                              Please push the back button and reload the previous window.'
                              An internal error has occurred and the module cannot be displayed.
                              There are no results that meet this criteria.
                              Search Result for "|||"