Piston weight difference

Big Block A body Tech

  1. mhuppertz

    mhuppertz Well-Known Member

    Messages:
    208
    Likes Received:
    24
    Joined:
    Dec 25, 2011
    Location:
    Tijeras, NM
    Local Time:
    8:27 AM
    I am prepping my parts for bob weight calculation to balance my forged crank.
    Is 1 gram max difference between pistons acceptable for a 440 that probably won't see more than 6k rpm or so, or do I need to zero them out?
     
  2. Krooser

    Krooser Reform School Graduate

    Messages:
    2,183
    Likes Received:
    1742
    Joined:
    Jul 19, 2015
    Location:
    Central Wisconsin
    Local Time:
    9:27 AM
    Run them... 1 gram is nothing compared to factory tolerances.
     
    • Agree Agree x 3
    • Like Like x 1
    • forphorty

      forphorty Well-Known Member

      Messages:
      661
      Likes Received:
      241
      Joined:
      Jan 19, 2006
      Local Time:
      9:27 AM
      A gram is .0357 ounce. How much does a drop of oil weigh?
       
      • Like Like x 1
      • Agree Agree x 1
      • RustyRatRod

        RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

        Messages:
        59,752
        Likes Received:
        30810
        Joined:
        Jun 7, 2010
        Location:
        Georgia
        Local Time:
        10:27 AM
        Well......you DO have it apart.
         
        • Like Like x 1
        • Agree Agree x 1
        • 72Dart6pack

          72Dart6pack Well-Known Member

          Messages:
          3,532
          Likes Received:
          3067
          Joined:
          Feb 1, 2016
          Location:
          Roseville Michigan
          Local Time:
          10:27 AM
          A paper clip weighs 1 gram. A 5 cent nickel weighs 5 grams. Just as a reference.
           
        • famous bob

          famous bob mopar misfit FABO Gold Member

          Messages:
          16,144
          Likes Received:
          6058
          Joined:
          Aug 14, 2011
          Location:
          okla
          Local Time:
          9:27 AM
          Why not try for perfect ? Since their in ur hands anyway ! Unless ur in a hurry .
           
        • toolmanmike

          toolmanmike FABO MODERATOR Staff Member FABO Gold Member

          Messages:
          48,436
          Likes Received:
          33054
          Joined:
          Jan 18, 2006
          Location:
          Iowa
          Local Time:
          9:27 AM
          Doesn't your machine shop do that for you?
           
        • Oldmanmopar

          Oldmanmopar Going left turning right FABO Gold Member

          Messages:
          8,023
          Likes Received:
          4027
          Joined:
          Jan 8, 2009
          Location:
          Danielsville Pa.
          Local Time:
          10:27 AM
          The closer to zero on the balance the smoother the engine which only increases power and longevity.
           
          • Agree Agree x 1
          • 72Dart6pack

            72Dart6pack Well-Known Member

            Messages:
            3,532
            Likes Received:
            3067
            Joined:
            Feb 1, 2016
            Location:
            Roseville Michigan
            Local Time:
            10:27 AM
            What happens when one piston is under weight? Do you add weight? Or take weight off the other 7?
             
            • Like Like x 1
            • toolmanmike

              toolmanmike FABO MODERATOR Staff Member FABO Gold Member

              Messages:
              48,436
              Likes Received:
              33054
              Joined:
              Jan 18, 2006
              Location:
              Iowa
              Local Time:
              9:27 AM
              Unless you have a crank balancer why bother. How are you going to balance that? (against the bob weight)
               
              Last edited: Jan 19, 2020
            • RustyRatRod

              RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

              Messages:
              59,752
              Likes Received:
              30810
              Joined:
              Jun 7, 2010
              Location:
              Georgia
              Local Time:
              10:27 AM
              Find someone with a paint scale. Measure all the pistons and find the lightest one. Get on the inside of the pin boss with a grinder and dress them until each piston weighs the same. It's not hard. Yeah it's "just" a gram, but how much is one gram throwin around at 6000 RPM? I don't know. If I had them all apart and in my hand, I'd get them dead perfect.
               
              • Agree Agree x 2
              • pishta

                pishta I know I'm right....

                Messages:
                16,897
                Likes Received:
                6568
                Joined:
                Oct 13, 2004
                Location:
                Tustin, CA
                Local Time:
                7:27 AM
                I had 7 pistons at x, and 1 piston and x-2g. I hit the inside of the pin with a Mig wire and built up the 2g there. The pins were floaters so EZ to slide out and weigh. I scrubbed the inside with a brush to make sure no slag was left in there. I took weight out of the pins in another case where I had 3 pistons that were >+5g out and there was not 5g in he piston to take out. Strange lot of pistons there.
                 
              • Machineman63

                Machineman63 Active Member

                Messages:
                36
                Likes Received:
                11
                Joined:
                Jul 20, 2017
                Location:
                Freeport Maine
                Local Time:
                10:27 AM

                Out of balance of 1 gram then add the dynamics of 2,000 rpm, that actual force squares at 4,000 rpm, then that force squares at 6,000 rpm.
                Your seat of the pants wont notice it, but in the motorsport theory of everything counts...maybe its worth it.

                I had a 71 340 engine with 150,000 mile on it and one rod was 42grams lighter than the heavy one, must have been down on power fighting off physics like that.
                 
                • Like Like x 1
                • Krooser

                  Krooser Reform School Graduate

                  Messages:
                  2,183
                  Likes Received:
                  1742
                  Joined:
                  Jul 19, 2015
                  Location:
                  Central Wisconsin
                  Local Time:
                  9:27 AM
                  I bought 18 SBC LT1 PM rods when we were gathering parts for my kids dirt modified.

                  The weights vary by 15 grams. These were nearly new rods that came out of marine engines that froze during the winter.

                  These rods are used by hundreds of guys in their dirt cars... Most engine shops screw em together and simply balance the cranks.. new pistons are pretty close on weight
                  Anyway that 15 grams doesn't seem to make much difference in these motors running 50 or so laps per week at 6500-7000 rpm.

                  My son's engine runs smooth and strong.
                   
                  • Thanks! Thanks! x 2
                  • EchoSixMike

                    EchoSixMike Well-Known Member

                    Messages:
                    268
                    Likes Received:
                    88
                    Joined:
                    Nov 20, 2013
                    Location:
                    Chicago
                    Local Time:
                    9:27 AM
                    You guys are creating stress risers in aluminum parts for nothing. And MIGing on heat treated wrist pins.....FFS. S/F....Ken M
                     
                  1. This site uses cookies to help personalize content, tailor your experience and to keep you logged in if you register.
                    By continuing to use this site, you are consenting to our use of cookies.