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Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

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  • Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

    I?m sorry to ask this question, but I forgot exactly what I?d learned in college. I?m building an increased stroke 280 Mercedes (1960s) engine, had the crankshaft offset welded, ground and balanced. I had wondered why the balancer didn?t need to know the weight of my slightly lightened piston and rod assembly, in order to remove the correct amount off the counter weights and he insisted that on an inline 6 cylinder there was no need to change the balance of the crankshaft unless I was building a high revving racing engine (I?m not/ 6500rpm). While I didn?t notice any imbalances after assembling and testing this 1st version of the modified engine, the current 2nd version has custom pistons, rings and wrist pins that weight approximately 150 grams less than the original. Does the balance now have to be adapted to achieve a smooth running engine? Ross Pistons and my crankshaft grinder (Marine crankshaft) claim ?NO??, but don?t seem to be able to back-up why...

    I seem to remember that there can be several ?orders? of frequencies that occur on any engine and while it seems likely that on an inline 6 cylinder, one of those orders is inherently balanced, if their were NO imbalances at all, on an inline 6, it wouldn?t make much sense that the manufacturer added fairly sizeable counter weights opposite most of the crank shaft journals, to balance the weight of (I?m assuming), the big end journal, the pistons and the oscillating mass of the connecting rods. They even take care to fine tune, (drill) these weights to achieve the accurate balance. This is what I am trying to understand. Why would it not matter if I change the weight of the pistons and rods.?

    Much thanks, in advance, Tom

  • #2
    Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

    ;D
    You're ok man, I understand worrying about a build, but what you're doing
    Is something that's been done to thousands of cars. I'd be more concerned that all
    the new stuff was equal weight relative to each other. Assuming you know the crank and rods are good.
    But with the amount of work you already had done to the crank I'd say your boys took
    care of you and got her close. If you've got a stock Flywheel and heavy dampener up front I wouldn't worry
    about it. Especially with out any rpm's over 7k, you're fine.

    Get back together and drive it.
    Welcome to the forum by the way tons of great guys here.
    You should ALWAYS put up pics of your projects.
    Even for this question guys love to see internals,block,anything. ;)

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    • #3
      Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pist

      I'm hoping some of the enginerds check in on this. I've always heard that inline sixes are naturally in balance but I never understood exactly why (or maybe "how"). I'll be following this thread. I'se juss' a technikan.

      Dan

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      • #4
        Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

        Originally posted by DanStokes
        I'm hoping some of the enginerds check in on this. I've always heard that inline sixes are naturally in balance but I never understood exactly why (or maybe "how"). I'll be following this thread. I'se juss' a technikan.

        Dan
        I think it's because they are 120* relation to each other. With the companions
        being in the same relation to the shaft always . When i pulled out my notes from school, I had in there
        That Specifically merc i6-no balance (that's all i had on it) so i would agree with you.

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        • #5
          Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

          This explains it more eloquently than I,I tend to get hung up in my words.

          An inline six engine is in perfect primary and secondary mechanical balance, which can be achieved without using a balance shaft. The engine is in primary balance because the front and rear trio of cylinders are mirror images and the pistons move in pairs. That is, piston #1 balances #6, #2 balances #5, and #3 balances #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because an inline six cylinder crankshaft has six crank throws arranged in three planes displaced at 120 degrees. The result is that differences in piston speed at any given point in rotation are effectively canceled.

          An inline four cylinder or V6 engine without a balance shaft will experience secondary dynamic imbalance, resulting in engine vibration. As a general rule, the forces arising from any dynamic imbalance increase as the square of the engine speed?that is, if the speed doubles, vibration will increase by a factor of four. In contrast, inline six engines have no primary or secondary imbalances, and with carefully designed crankshaft vibration dampers to absorb torsional vibration, will run more smoothly at the same crankshaft speed (RPM). This characteristic has made the inline six popular in some European sports-luxury cars, where smooth high-speed performance and good fuel economy are desirable. As engine reciprocating forces increase with the cube of piston mass, inline six is a preferred configuration for large truck engines.

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          • #6
            Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

            Even though the straight 6 is in balance, there is still a significan stress on the crankshaft if it does not have counterweights. That's why it has counterweights, which balance out each section of the crankshaft, so it will not be trying to bend all the time.

            at least that's my understanding
            My fabulous web page

            "If it don't go, chrome it!" --Stroker McGurk

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            • #7
              Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

              Originally posted by squirrel
              Even though the straight 6 is in balance, there is still a significan stress on the crankshaft if it does not have counterweights. That's why it has counterweights, which balance out each section of the crankshaft, so it will not be trying to bend all the time.

              at least that's my understanding
              Right so it's balancing the weight of its self as well as the rod,pins,pistons. So if it's all in harmony and
              the weight is reduced only on the "Bob weight"side as long as it's equal to each other than that it's the same
              dynamic at the crank, just less weight.

              Where's one of those boys from Spectre when you need em. :D

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              • #8
                Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

                always get it balanced

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                • #9
                  Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

                  Wow. Thanks guys, for the fast and good answers. I will send pictures as soon as I get the crank back in. We all agree that an in-line six doesn't need a separate balancing shaft and that it is inherently balanced because of the 120 degree off set and the motion of the pistons (although my engine has 1 and 6 traveling together, 3 and 4 etc.)

                  But there is STILL the issue of the large counter weights on the opposite ends of the crank pins. They are there for a reason. I think Squirril's post comes close to what I am trying to answer. If I remove weight from the piston and rod, it seems like there will be too much weight in those counter weights and I would need to remove some. While apparently
                  both the first and second order is inherently in-balance in an in-line six, it looks like the weight opposite the crank pins may be required to reduce imbalances locally at each journal, even though the crankshaft as a whole is still in balance. In theory these stresses might still want to (very slightly) deform these individual crank journals, while the whole thing still runs smoothly....
                  By the way, the crankshaft grinder did "balance" the crankshaft, but I believe this was simply a static balancing of the whole assembly and he is one of the guys who said don't worry about the rest at the speeds your running. This may be true, but I'd still like the correct answer and obviously there must come a point where I've reduced the weight enough so that the counter weights would become an issue. Right now I've reduced the weight almost 20%.

                  Thanks, Tom

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                  • #10
                    Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

                    Good luck man,

                    Hey while it's at the shop have them put the
                    BOB weight of the new parts on it and put this to rest.
                    I'd like to hear the final word. That should be a quick
                    answer, either she's in or out and needs weight removed.

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                    • #11
                      Re: Does an Inline Six Crankshaft have to be Re-balanced for Lighter Custom Pistons?

                      I think the front and rear are there for torsional stability and to dampen the high power consumption (compression) and high power output (power) stroke pulses by giving the crank enough mass to absorb the inertia changes without bending.
                      Central TEXAS Sleeper
                      USAF Physicist

                      ROA# 9790

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