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  • #16
    I think some of you are confusing balancing V8s and V6s where you have to add bob weights, to balancing inline 4s and 6s where you dont need to. As long as the rods and pistons all weigh the same weight (+/- 1-2 grams) then that is all you probably need . You can balance the rotating assembly c/w flywheel, clutch and front pulley
    if you wish, but unless you are going to use it for competition then the factory limits are probably ok. Normally they will balance out the flywheel and clutch and the crank seperately, then bolt them together and recheck. There is no internal or external balancing on a 4 cylinder motor. You do it by adding or removing weight from the crankshaft counter weights.
    Grelley

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    • #17
      Originally posted by Jake View Post
      vmwerks/inastrangeland,

      Do you guys go all the way when you balance? I'm under the impression that for our engine most do not weigh each component and add bobweights as descibed above, just spin balance the crank/flywheel/damper.

      Jake
      Yes I have both the reciprocating and rotating assemblies balanced. I have the rotating assembly balanced from the balancer/pullies to the clutch disc and pressure plate. Both ends of the rods are matched; the pistons, rings, wrist pins and clips are all matched among other things. When they are done you should get a balance sheet with all the numbers on it. There is simply no reason not to do it. Everything will last longer, the engine revs more smoothly and quickly. The cost is minimal compared to the rest of the engine rebuild, you will notice the extra cost.

      Use a machine shop that balances in house as some send them out to be done. Do it half way or all the way the choice is yours but there's a reason why there are balancing pads on your rods.
      ________________
      2010 335d
      1999 E46 U.S. Touring Car
      1988 E30M3 S52

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      • #18
        Originally posted by vmwerks View Post
        Both ends of the rods are matched; the pistons, rings, wrist pins and clips are all matched among other things.
        And that's the point I'm trying to make vmwerks. What you have described is NOT a complete rotating/reciprocating balance as I explained. You are not adding bob-weights and adjusting crank counter-weights based on reciprocating weight to compensate for lighter rods and pistons. Matching rod and piston weights between each other is not the same as adding calculated bob-weights to the crank and THEN balancing on a machine.

        The confusion may be unintended, but it falsely leads the customer to believe he's getting something more than the basics. Go back to the original question.


        Originally posted by davem3 View Post
        I am a new evo 3 crankshaft and other pistons (lighter + - 430 g) connecting rods and lighter (600 grams)

        I must now again to balance my crank on the weight of what the pistons and connecting rods are ?
        Ultimately he's asking if the crank counter-weights have to be re-balanced to the new lighter piston and rod weights. The short answer is no. And per your explination vmwerks, you don't do this either. All you're going to do is match rod and pistons weights between them in addition to the normal rotating balance procedure. This is all I do on my engines.

        Jake

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        • #19
          Jake I never said they didn't use bob weights. Bob weights are not in the engine when assembled. I was only clearing up what we have done when we send our engiens out for machining.
          ________________
          2010 335d
          1999 E46 U.S. Touring Car
          1988 E30M3 S52

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          • #20
            I don't mean this as a put down vmwerks, but I don't think you are clear on what the question is.

            Of course bob-weights are not installed in the engine when assembled. I'm not sure why you had to point that out. They are a balance tool to be removed after balancing.

            What you have not been clear on is if you use them when you balance your engines. You have also not been clear on whether or not you adjust crank counter-weights with respect to lighter pistons and rods. Do you do this?

            Jake

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            • #21
              I am going to add to the confusion.....

              1. When I built my engine the guy at the machine shop tried to talk me into balancing the crank/pulley/flywheel/clutch as an assembly. I decided against it. What happens if you want to change one of the components? Makes more sense to me to balance the parts individually (they are probably close as they come from the factory) then as long as anything you add is balanced on its own....changes to parts should not be a problem...nor should orientation when they are bolted on. Most engines will outlast the clutch attached to them....for example.

              2. I think there is some misunderstanding here about the function of crank counterweights. "I think" they are solely designed to counter the weight of the rod journal so the crank rotates as a balanced rotating mass. They are not intended to be balanced with the mass of the pistons/rods.

              3. I don't pretend to be an expert, but I think on a 4 cylinder engine bob weights are not used in balancing. If the crank is balanced as a rotating mass....piston assembly weights are equal....and rod end weights are equal...a 4 cylinder engine is considered to be balanced. Elaborate measures such as bob weights are not necessary because of the design of the 4 cylinder engine....everything is in direct opposition to other parts. So if you buy high quality parts, a 4 cylinder engine could quite possibly be bolted together out of the box and be perfectly balanced. In my case, I found the rods (Pauter) were perfectly balanced as they came from the factory, but the 2.5 crank needed to be "perfected" and I had to remove some piston material to get their weights to equal out. But there is no reason the parts could not have been sold as a set "already balanced".

              4. I don't entirely understand it....but I think spinning the crank with bob weights is only necessary when balancing "V" configuration engines. Something about the dynamics of how they operate requires it.

              Again, I am no expert....but this is the way I understood it.....

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              • #22
                Originally posted by grelley View Post
                I think some of you are confusing balancing V8s and V6s where you have to add bob weights, to balancing inline 4s and 6s where you dont need to. As long as the rods and pistons all weigh the same weight (+/- 1-2 grams) then that is all you probably need . You can balance the rotating assembly c/w flywheel, clutch and front pulley
                if you wish, but unless you are going to use it for competition then the factory limits are probably ok. Normally they will balance out the flywheel and clutch and the crank seperately, then bolt them together and recheck. There is no internal or external balancing on a 4 cylinder motor. You do it by adding or removing weight from the crankshaft counter weights.
                Grelley
                This is EXACTLY what the guy who built my motor told me and he is about as highly regarded as I know in the area. People from all over the country (mostly down South) ship motors to him for building and custom work.

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                • #23
                  ^ I agree with this procedure. In the motors I've built, I gram balance the individual set of parts pistons, wrist pins, rods (weighing both ends) even though my Pauters were balanced I had to clearance the small end. Then the flywheel, clutch, pressure plate, dampener individually and then dynamically together with the crank from the dampener bolt to flywheel.

                  I suppose id the dynamics of V-engines necessitate bob weights, the angle of an inline engine could probably be done but I doubt it would yield much if any benefits.
                  Rich!

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                  • #24
                    Originally posted by Jake View Post
                    I don't mean this as a put down vmwerks, but I don't think you are clear on what the question is.

                    Of course bob-weights are not installed in the engine when assembled. I'm not sure why you had to point that out. They are a balance tool to be removed after balancing.

                    What you have not been clear on is if you use them when you balance your engines. You have also not been clear on whether or not you adjust crank counter-weights with respect to lighter pistons and rods. Do you do this?

                    Jake
                    Since I am not the machinist I speak from a customer point of view. I have been taken on a tour of the shop that does my work and all the balancing for most of the local machine shops, including some well know engine builders. I was shown what exactly was done to the race engines that we build. When you send your parts to the machine shop they weigh ewverything and adjust accordingly. On the crankshaft counter weights they drill out material as necessary. I would imagine that if you're going to use lighter components they would. However that said, I've seen them remove material from a counterweight that for an engine built with all OEM components.

                    You asked why I did not mention bob weights, my answer was, they do not belong in the engine. You pointed out that I never mentioned the bobweights in my list of components to be balanced. For me, as a customer of a machine shop, it's up to them, they are the professionals.

                    The original question said nothing about bob weights, just do I need to balance my engine since I have lighter components. My answer was YES always balance the entire engine since the cost is minimal.

                    No offense taken and none meant... this thread was going off in directions that had little to do with the original post of "should I or shouldn't I" balance. I said yes and gave reasons why...
                    ________________
                    2010 335d
                    1999 E46 U.S. Touring Car
                    1988 E30M3 S52

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                    • #25
                      Reading...

                      http://vibrationfree.co.uk/publications/

                      Steve
                      Sport Evo No.47

                      My Sport Evo Restoration

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                      • #26
                        Originally posted by stevesingo View Post
                        None of the links work for me... :(
                        ________________
                        2010 335d
                        1999 E46 U.S. Touring Car
                        1988 E30M3 S52

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                        • #27
                          I read his original question differently. It wasn't a general "should I balance my engine". He was specifically asking about his lighter rods and pistons with respect to changing the weight of the crank counter-weights.

                          That's a big question for me as well which is why I'm interested in this thread. The actual motorsport crank is lightened slightly. I always assumed the counter-weights were trimmed to match a certain percentage of the lightened motorsport rods and pistons. Just so we're clear, the material removal was beyond simply balancing the crank, the counter-weights were purposely trimmed down (not removed entirely as some have done). The change was subtle but obvious.

                          So that drives the question. If you are using lighter rods and pistons, should you trim the counter-weights as well? If so, by how much? How is it calculated? As long as I've been involved in the S14 I've not seen this done or heard any recommendation to do so. I have seen cranks with the counter-weights nearly removed entirely, but this is well beyond simply adjusting counter-weights with respect to lightweight rods/pistons.

                          If balancing an inline 4 cyl engines does not require the use of bob-weights, and if crank counter-weight and reciprocating weights don't correlate, then what's the point of the counter-weights in the first place? If each cylinder assembly is balanced to each other, and they are equal-opposites and therefore balance themselves out dynamically, then what originally determined crank counter-weight mass?

                          To me, it was obvious to assume reciprocating weight and crank counter-weight had some kind of correlation. But, in so many words, some here have indicated that they don't. I also assumed that the correlation (if any) was not critical because there have been many built engines running lighter rods/pistons and standard unmodified cranks (aside from the usual balancing) without issues. The only real problems I'm aware of were with overly lightened cranks and/or aluminum/undampened crank pulleys cracking blocks and even breaking the cranks themselves.

                          Lets get some pictures going.

                          Originally posted by GruppeAm3:

                          This shows two 2.5 cranks. The crank on the left is a real MS version. The one on the right is the standard street version. If you look closely, you can see that material has been removed from the counter-weights on the MS crank.






                          And here is an example of a crank which has gone beyond simply matching new light weight reciprocating assemblies.






                          Jake
                          Last edited by Jake; 07-08-2012, 04:12 AM.

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                          • #28
                            hi

                            yes the question was
                            I have to balance the crankshaft again
                            because I'm lighter pistons and connecting rods

                            the counterweight of the crankshaft change

                            So what jake says


                            sorry for my bad english


                            regards dave

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                            • #29
                              Yes. If the rotating assembly has already been done it isn't 100% necessary you can just gram balance the pistons and rods... but since it's not expensive it's best to just redo all of it.
                              Rich!

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                              • #30
                                Originally posted by ///schwartzman View Post
                                Yes. If the rotating assembly has already been done it isn't 100% necessary you can just gram balance the pistons and rods... but since it's not expensive it's best to just redo all of it.
                                I think Rich is on point here...just redo it but I'd weigh the pistons and rods and be done. Unless you are are running an all out race motor...it's probably going to fall under the "overkill" category but if you want a motor that has had the total rotating assy balanced/weighed it's up to you and as many state, likely little added cost all things considered.

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