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  • #16
    Originally posted by simplyconnected View Post

    2. A complete piston assembly called a 'sample' must be submitted with the crankshaft for balancing. We bolt the caps to the rods to keep the bearings in place. We install wrist pin keepers to hold the piston (with rings) and rod together to make a sample piston with all its parts. - Dave
    So, your saying that you hand your machine shop executing the dynamic balancing of the crankshaft (or, as is often referred as to "spin" the crank) one assembly, as you refer to as a "sample", consisting of piston, ring set, gudgeon pin w/ locks, connecting rod, w/ bearings inserts secured, as a singular assembly? And he (not being sexist, just most often in this business) does what with it? This was, and still is, my question.

    And yes, I am familiar with most of the equipment and accepted procedures involved in the endeavor, just never experienced this procedure. But, as I stated previously, there may be more than one way to skin the cat, but, just make sure you get the right cat!



    • #17
      Originally posted by pbf777 View Post
      ... just make sure you get the right cat!..
      Or in this case, since the crankshaft was balanced for OEM parts, make sure the crankshaft is re-balanced if different-weight parts are used.

      It's just exactly the same as when new tires are put on old rims. The only difference is, we can remove more weight from a crankshaft but adding it back requires more work. Those counterbalance (throws) are there to counteract forces (or mass) imposed by the pistons.

      If the new pistons are heavier, the crankshaft counterbalance must also be 'more heavy'. In other words, we have to ADD weight to the crankshaft counterbalance. We do that by drilling a hole and welding Mallory Metal into the counterbalance weights (or throws).

      If the new pistons are lighter than OEM (which rarely happens) we simply drill or grind more weight out of the corresponding counterweights.

      How do we know if more or less counterweight needs to be added or removed? With a 'sample piston', we replicate that weight into the bob-weights, bolt them onto the crankshaft and 'spin it'. If the crankshaft is beyond the limits of balance, in other words the pistons are 'too heavy' for the counterweights, more weight must be added to the counterweights. This usually adds another hundred bucks to the cost of the job.

      I've been there with a Y-Block I bored .060" over. The replacement pistons were much heavier, Mallory metal was added, the crank was properly dynamically balanced and now it runs smooth as silk and MUCH stronger.

      If you still have questions regarding the 'sample piston' please ask. Never rely on the original balance job. It was done in mass production using archaic methods before computers existed. - Dave

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      "We've got to pause and ask ourselves: How much clean air do we need?"
      --Lee Iacocca