Question about balancing rotating assembly

Small Block Mopar Engine

  1. nm9stheham

    nm9stheham Well-Known Member

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    And BTW, there may be some confusion between some terms that leads to misconceptions: 'Damper' and 'balancer' and 'harmonic balancer'. Assisting balancing with that 'thing' on the front of the crank is one function. Damping the internal high frequency vibrations (harmonics) is a separate function (though it really is not a harmonic if I understand it right). That 'thing' on the front of the crank can do either one or the other, or both.

    Folks tend to use all 3 terms interchangeably but that leads to confusion on what it actually does and perhaps helps make folks think that damping has something to do with balance. The 273, 318, 5.2L and early 340 engines only have a 'damper' function up front. The external 360 and 5.9L and late 340 (external balance engines) have both functions, so the term 'balancer/damper' (which nobody uses) or 'harmonic balancer' is the right term.
     
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    • yellow rose

      yellow rose Overnight Sensation FABO Gold Member

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      I went with a friend to buy a “race” KX 500 Kawasaki because I have ridden big bore 2 stroke dirt bikes since 1981 and he wanted an extra set of eyes.

      We get there and the seller is leg humping my buddy to death about how the pro who did the engine was the guy who built and tuned for so and so and crap like that.

      The bike was clean, but the proof is in the riding. Since I was short and fat, the dude wanted to start the bike himself, but I’ve been short and fat for a long time and I wanted to make sure I could start the pig so I could teach him to start it.

      Once it was running I let it warm up a bit, shut it off and said we pass. The seller was wetting himself because my friend was a cash buyer and the seller was hard up. I said we will pass again. Then the seller say “you said it’s all about the ride and yet you won’t even ride it”. So I said my buddy will ride it, because it’s going to be his bike and I don’t want to have to ride it because he hates it.

      The seller says if he’s a rider he’s a buyer. So I took my buddy aside and said “did you see how smooth that thing is at an idle” and he says yep, I think that’s a good thing. So I told him to remember that when he rode it.

      He made one lap around the field, came back and said we pass. The seller was livid. He wanted to know what I said to make him not buy it. I said he can tell you ne my buddy says “it’s too damn smooth at idle”.

      And we walked away.

      When we got in the car he said how did you know it would shake like that? And I said when dealing with big bore two strokes if they don’t shake a bit at idle, they’ll shake your head off in the upper rpm range.

      And it did. It was so far out of balance you couldn’t even ride it. Because the guy who did the work didn’t know or understand you balance those things at 60% or more over balance. If you don’t do that, they are smooooooooth at idle and will throw you off the bike if your not careful.
       
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      • nm9stheham

        nm9stheham Well-Known Member

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        That's good YR! LOL
         
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        • dartfreak75

          dartfreak75 Restore it, Dont part it! FABO Gold Member

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          I did extensive research on this topic along with the help of @nm9stheham. After being told I had to get a rebalance when changing the pistons. I did a lot of reading and talking with guys and even the machinist that is doing my machine work all told me for a street car it's not necessary. I bought a book from a member here and even it says its not necessary. I talked to a guy I work with who is a big mopar nut like me and he said he has been building mopar small blocks for 30 years and has never balanced one and has never had an issue even in his race car. He told me he built a 360 very similar to my build except he used kb pistons instead of speed pros and he did not have it balanced! He said he sprayed it with a nitrous and it was running in the 10s in a 68 barracuda. I have no personal experience with this just a lot of hours talking and researching but here soon I will when I get my engine back together. But from my research I'd save the money for something else and send it!
           
        • gregcon

          gregcon Well-Known Member

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          As for the V-10 damper issue etc.....I'm well aware a lot of engine have the damper very well secured. I'm also aware a lot of engines have them far less well secured. For example, the mighty Slant Six - made by the millions in a hundred applications - in almost every case used no bolt at all. They just pressed the damper in place and off it went.

          Did the V-10 damper come loose and allow the problems to happen, or did the problems cause the damper to come loose? I don't know.

          So back to my point...the damper is there to dampen harmonics...but to suggest that the damper soaks up all the issues related to harmonics, 100%? No way. It's like a lot of parts - it does a job to a largely successful degree, but it doesn't do it perfectly.

          In any case, this thread has had a lot of good inputs from a lot of different people and they all pretty much verify the same thing - when someone walks into a machine shop and orders up a 1/4 gram balance, they are fooling themselves and helping the machine shop owner put his kid Lewis through medical school.
           
        • nm9stheham

          nm9stheham Well-Known Member

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          Now you've put something in my mind that I am going to have to think about... LOL. There has to be a good explanation.
           
        • 66jim

          66jim Well-Known Member

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          Torsional harmonics change with engine speed,maybe go with a fluid damper, more money but more effective. Match the internal parts as best you can.
           
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