Question about balancing rotating assembly

Small Block Mopar Engine

  1. jcwren

    jcwren Member

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    I'm rebuilding a 318 engine that I don't know the history of. The block casting is 2536030 ('67, 0.030 over), the crank casting is 2658393, and the connecting rod castings are 2406782. It's all been to the machine shop and cleaned up. Rods have new ARP bolts. I weighed the the rods (with bolts, but without bearings), and the lightest is 724 grams, the heaviest is 736 grams. I did not build a jig to weigh the large ends and small ends. Pistons with rings have a 2 gram spread. My scale has 2 gram resolution, so I know if I do anything myself I need a better scale first.

    How bad is a 12 gram spread? Is this worth having the shop balance? While I'm trying to do this right (the operative word there is "trying"...) I don't want to spend money needlessly. I'm not building a 10 second car here, but I'd like to leave myself some room for improvements down the road.

    Thanks!
     
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    • perfacar

      perfacar Well-Known Member

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      12 grams won't affect balance. I was told 30 grams and up yes it will affect balance. if it's lighter, that's very ok.
       
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      • Krooser

        Krooser Reform School Graduate

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        You'll be ok...millions upon millions of engines have run perfectly with those tolerances. I raced for 20 years before I had an engine balanced.

        Here come the slings and arrows!
         
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        • nm9stheham

          nm9stheham Well-Known Member

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          Sounds right.. I have 728 grams as the nominal weight for those rods. That is indeed the normal spread of factory weights; I weighed a set with that much spread in rod weights.

          You will probably have a bit better overall balance by putting the lighter rod with the heaviest on one journal, match the 2nd lightest with then 2nd heaviest on another journal, etc.

          Were the rod ends resized after the ARP bolts were put in?
           
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          • jcwren

            jcwren Member

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            Thanks for replies.

            I had found information in my books and online about balancing, but I didn't find any numbers about what was out of spec. I suspected the 12 grams was acceptable since those were the original rods, but never found a comment about "xx is too much!".
             
          • jcwren

            jcwren Member

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            Yes, they were resized. Not sure I want to move the rods around since they're numbered. I will check to see how they're paired up when they're installed.
             
          • toolmanmike

            toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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            There is probably no spec. The rods are all weighed and all lightened to the weight of the lightest one. All the reciprocating weight is added to the crant throws (bob weight), balanced on a machine and weight is added or material removed from the crank throws to balance the assembly. You can "weight match" the rods but it's not balancing the engine.
             
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            • yellow rose

              yellow rose Overnight Sensation FABO Gold Member

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              ^^^^^^^^^^^THIS^^^^^^^^^

              Balancing a crank is not just science. The very best there are that do it don’t agree on much anything. There is (just to name a few) things that make balancing a crank as much an art as a science, and so many things affect balance such as crank materiel, block materiel, the way the crank is made, the rods you are using, displacement (yes, it matters) RPM (that’s a biggie if you want to start a fist fight about crank balancing...BTDT) and other things.

              There is the standard 50% balance job, which even the best minds have a hard time determining where that number came from. I suspect it came about empirically, but can’t say it for fact. There is over balance (51 or 52 or more % of the reciprocation weight IIRC) and there is under balance (49 or 48 or more % of the reciprocating weight IIRC) and THEN you move on to the argument about WHEN to use over under or 50% (be ready to fight if you have a definitive answer because people have their ways and they don’t always agree) or if you should ever use it.

              The fact is for most street or street/strip stuff if you get the rod ends to under 2 grams difference and get the crank under 20 grams out on either end it will be 100 times better than the factory balance.

              FWIW, on my street/strip stuff I was under a gram on the rod ends and under 5 grams on the crank at 50%. On steel rod race stuff I was under a half a gram on the rod ends and 2 grams on the crank at 50%.

              If you were running 400 inches or less (a totally arbitrary number I pulled out of my hat based on general small block Detroit engines) or 500 inches or more (again, an arbitrary number I based of basic BB Detroit architecture) the I would look at the Bob weight and power output and maybe use the above race numbers and add a percent or two based on the Bob weight.

              FWIW on my aluminum rod stuff I personally ran and shifted at 8500-8800 I was at 53% over balance. Every time I looked at the bearings after a season, the more over balance, the better they looked. And that’s basically how you determine how you balance a crank. It’s like making hot dogs.....
               
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              • rustycowll69

                rustycowll69 Well-Known Member

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                I took apart my relatively virgin 70 340 six pack engine, and although I can't remember the exact numbers, because that was over 40yrs ago, I was shocked how different each rod was. I think it was like 80 to 100 grams(if not higher) off from highest to lowest. I never noticed any bad vibration problems.
                 
              • yellow rose

                yellow rose Overnight Sensation FABO Gold Member

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                Yep. Seen it myself. I’ve been told chrysler used what is called mass balancing. I’ve asked some of the smartest people in the industry to explain it, but no one could.

                You can be pretty far off with the counterweights being heavy and it won’t hurt most Street/strip duty stuff. When the counterweights get light...hang on as they shake like a dog pooping peach seeds.
                 
              • jcwren

                jcwren Member

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                I had no idea that a dog pooping peach seeds would shake :)

                All that being said, I was primarily concerned because I don't know the history of the engine, and although the rod casting numbers were all the same, I can't say for absolutely certain that someone didn't swap rods from another engine and they weren't originally closer in weights. While I can weigh rods and pistons and rings, cranks are a little harder. And I'm told trying to spin balance it with an air wrench is a bad idea :)
                 
              • nm9stheham

                nm9stheham Well-Known Member

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                Just looked at my data from a light rod set out of a 273.... 10 grams heaviest to lightest. 10 grams variation on the small ends and 12 grams on the big ends.

                And bobweight variation across all 4 journals with the original parts (pistons, rings, rods, bearings etc.) as installed in the factory was 11 grams.

                Re-arranging the rods is just a suggestion... it may be very slightly beneficial but just as likely not. So no big deal.
                 
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                • nm9stheham

                  nm9stheham Well-Known Member

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                  Maybe some other parameter? I checked an original rod set from a 340 and got 5 grams heaviest to lightest. 4 grams variation on the small ends and 3 grams on the big ends.
                   
                • rustycowll69

                  rustycowll69 Well-Known Member

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                  I don't know what you mean. If you mean, was I mistaken? absolutely not. I wrote the numbers down at that time, but my kids will probably find it when I've passed. I bought a lab balance scale, and made a fixture to weigh each end, and brought them within a few grams. I figured it couldn't hurt, don't know that it made any diff since I never had it professionally balanced.
                   
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                  • gregcon

                    gregcon Well-Known Member

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                    When you are a normal dumb-ass, you sit around arguing about which spark plug or motor oil will turn your car into a Top Fuel dragster. When you start to feel smarter, you start espousing the merit of equal length, tuned exhaust systems and camshaft profiles. Then, when you are an expert you are confident enough to start the balancing and rod/stroke ratio discussions.

                    So anyway, just for the sake of mentioning it....I owned a '70 340 Swinger that I bought from the original owner in 1986. It had about 113K miles on it and ran great. There is absolutely zero chance anyone had ever been into the engine. When I did take it apart finally, I found one journal had 318 rods instead of the 340 rods like the rest of the engine. Significantly lighter. I did a lot of reading up in Hot Rod and Car Craft and determined the engine had to be shaking violently, even though it showed no signs of it and ran as smooth as butter.
                     
                  • yellow rose

                    yellow rose Overnight Sensation FABO Gold Member

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                    So you’re ok to pay someone to balance the rotating assembly and they do that?

                    Luck would have it my shit would shake. Can’t tell you how many balance jobs I’ve had to correct. A bad one was a BBC that the moron who owned changed out the fly wheel from external to internal balance and didn’t think to even look at them before he swapped them out.

                    I forget how far off that end was, but I couldn’t even spin the crank to 100 RPM before the crank was jumping out of the machine.

                    I call the idiot and verify that was the FW he used in the car. He says yep. I say did it shake? He says smooth as butter.

                    He was either lying or stupid, because I had him bring in the main bearings because his dumb ass took it apart and sure enough, the mains showed distress.

                    But it was smoooooooooooth as butter. Right.
                     
                  • gregcon

                    gregcon Well-Known Member

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                    What I'm saying is people talk about engine balancing as if it is an exact science that is achieved by 'their machinist' if they ask for it. And the more they spend, the better the balance. I used to get a kick out of guys who would spend 'extra' to get the engine balance within 1/2 gram instead of the standard 1 gram.

                    Most balance scales aren't that accurate - certainly not the beat up ones I see in auto machine shops. And most of them, if you weigh a part 10 times, will give you 7 different values. Which is correct?

                    Then there is the issue of bob weight....you can change that at will and arrive at some decidedly different balances. Who knows which is perfect? None are perfect.

                    You can be in 'perfect' balance at one RPM and out of balance at another. Which do you use?

                    And who knows how to assess the exact amount of weight of the oil that is sticking to everything? No one here.

                    Who knows how to figure in the role of torsional harmonics? Don't forget, the engine is connected to a long skinny line of gears and tubes and rods, all rotating. No one here knows how, that's who.

                    Who knows how to figure in the effects of harmonics from the combustion events? No one here.

                    All those things play a role....but we're gonna instead worry about weighing everything down to 1/2 gram.


                    The bottom line is any reciprocating assembly - such as an engine - is going to abide by the old adage 'good enough' when it comes to balance. Not 'perfect'. Just 'good enough'.

                    Want proof? Look under the hood of the most expensive Lexus, Rolls, Mercedes, etc. Those guys live and breathe smoothness....if anyone knows how to balance and engine, it's them. Do you see their engines solid-mounted to the chassis? Hell no. They all have very sophisticated and expensive mounts that use rubber, silicone, springs, and all sorts of tricks to obtain smoothness. They wouldn't need to do that if the engine was in perfect balance.
                     
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                    • yellow rose

                      yellow rose Overnight Sensation FABO Gold Member

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                      LOL...I wish you’d have said that in the first place because I not only agree with it, I said most of that in an earlier post!
                       
                    • nm9stheham

                      nm9stheham Well-Known Member

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                      Interesting.... Which journal? The exact journal makes a significant difference difference in the effect on primary balance.
                       
                    • gregcon

                      gregcon Well-Known Member

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                      Been too long, I don't recall. But it was not the center two, it was either 1/2 or 7/8.
                       
                    • nm9stheham

                      nm9stheham Well-Known Member

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                      Roger... the end journal weights have about 3x the effect on primary balance versus the weights on the center 2 journals.

                      BTW, torsional vibrations in the crank have nothing to do with balance, nor are they any consideration in the balance work that we do here. It is at a very much higher frequency, like 6000-8000 cycles per second (Hertz) versus the primary balance vibration that would be at 100 cps at 6000 RPM.

                      In reality, a few of us know 'a bit' more about this than are being given credit...
                       
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                      • gregcon

                        gregcon Well-Known Member

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                        I'm not suggesting no one knows anything...I'm just saying that we all know less than we'd like to think. I do believe there is an impressive base of knowledge 'in the world'...but mostly it exists behind closed doors in places that have the chops to create it. Lexus, Ilmor, GM, etc. Sometimes that information leaks out in one form or another and we get to get little nibbles of it.

                        Harmonics? I dunno. When they first brought out the V10 in Ram trucks, they had an failure point where the harmonic damper would wobble loose. The blame, according to Dodge, was driveline harmonics. I take that mean the engine itself was happy but when you bolted it to the rest of the driveline, it got unhappy.
                         
                      • yellow rose

                        yellow rose Overnight Sensation FABO Gold Member

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                        I agree. Resonance frequency is a hard task master and what is known is closely guarded. When you hear an engine on a dyno the first time it scares the hell out of you. They sound worse than a thrashing machine and sound like at any second it’s going to grenade.
                         
                      • Mopar Sam

                        Mopar Sam Well Fed Member

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                        I balance a lot of single cylinder Kohler 16, 14, 12, horse motors. A lot of them are strokers with counter weights so big they have to bolt on because they won't fit through the hole in the block.

                        It is definitely not an exact science. The last one That I did, had already been balanced at another shop. The guy said it would shake his fillings out. I weighed everything, made up the bobweight and spun it. It was only about ten grams off, and that is subjective, as there are different schools of thought on what percentage to use.

                        I corrected it and when he picked it up I told him that I probably didn't do him any good.

                        He called a couple of days later and said it was great. I was amazed, 10 grams is not much, but they really zing those little motors. Garden tractor pulling.

                        I use 65 Percent on the bob weight in the bigger bore motors, a little less, but never less than 62 percent in the smaller stock class motors.

                        The one nice thing about those motors is that you don't have to match weigh when there is only one of everything. it is also easy to put in mallory when you can unbolt the counter weights.

                        This doesn't help the OP, but just adding to the fact that balancing is not an exact science and sometimes you have to rely on what you have found to work.
                         
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                        • nm9stheham

                          nm9stheham Well-Known Member

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                          The damper is there to help absorb/soak-up the crank torsional vibrations (the much higher frequency twisting vibrations in the crank), and has to be VERY firmly clamped to the crank snout to do that. Again, not addressed in balance work but in the inclusion of a damper.

                          In your V10 story, the crank was actually not 'happy' without the damper. It is very 'unhappy'. The damper was designed in from the get-go to make it 'happy'. The damper fastener worked loose for some reason; it could have been a weak or undersized fastener, too thin a clamping washer, too low an installation torque, or several other reasons. That is why on the SBM and BBM, you have a BIG bolt in the crank snout, with a THICK washer used, and it is torqued to 135 ft lbs. That is all done to clamp the damper HARD to the crank, so that the damper can effectively soak up those internal, high frequency vibrations.

                          There are plenty more 'nibbles' available if you want LOL. I actually agree with a lot said about accuracy and the looseness in numbers. But some us have striven to get pretty good at those matters and have the backgrounds and know-how. The separation of big and small end weights is the big source of error IMHO, and few understand not just the 'how' to do it well, but also the 'why' of doing it the way it is done.
                           
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