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  1. #1
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    Engine balancing and disaster - help! :-(

    So I dutifully followed the piston brochure recommendations and heat-fitted the piston pin through the eye of the connection rod. My pins are centered and the pistons pivot smoothly and look great.

    Now to get the balancing done.... and the machine-shop tells me they can't balance the assembly UNLESS I PRESS OUT the wrist pins from the pistons, so that they can weight each part individually.

    Is this some kind of cruel joke?

    Surely these guys should be able to determine the bob weight and reciprocating weight on an assembled piston?

    hELp!

    ]Paul

  2. #2
    Member ZJ-John's Avatar
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    The first step in balancing an engine is weight-matching all the pistons and rods. To do this, all the components are weighed. The entire piston assembly, complete with rings and locks, can be weighed together, but the rods must be weighed in two stages. The small end and the big end of the rod must be weighed separately, since the small end falls under the reciprocating weight category while the big end is rotating weight.
    "weighed together", means not assembled.

    Calculating Bob Weights

    Calculating bob weights isn't difficult, but it does require a little time at the scales. Here's the complete list for determining your bobweights for zero-balancing a crank.
    Rotating Weight

    • Big end of rod (including fastening hardware)
    • Bearing
    • Oil (normally estimated at four grams)


    Reciprocating Weight

    • Piston
    • Wrist pin
    • Pin locks (if used)
    • Small end of rod
    • Piston rings


    Bobweight = Rotating Weight + (Reciprocating Weight x .50)

  3. #3
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    Mathematically, it is possible to calculate both the rotating weight and the reciprocating weight by a few simple measurements of the assembled piston+rod.

    So I'm extremely surprised that there is no option to do this on a balancing machine.

    In any case, the shop said I should press out just one piston, and they would weigh just one piston, pin, and rod. Because all my pistons are almost identical in weight, they would be able to work out the remaining numbers.

    (This is not going to be a high RPM engine.)

    I managed to turn a press-tool in my lathe and used it to press out one piston (without damage -- shew *sweating*). So I'm good for now.

    Any opinions?

  4. #4
    Member dukefromthecave's Avatar
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    Everything I read on the topic stated pretty specifically not to assemble rod/piston until after balancing.
    I actually used stock style pistons and did not balance. Reused harmonic balancer too. Feel free to laugh at me if I need to spend hours disassembling when it runs like a jackhammer...

  5. #5
    Modfag Saleen4971's Avatar
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    weights vary - they need to be disassembled so that the natural balance can be as close to neutral as possible.
    Ross
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  6. #6
    Member SilverXJ's Avatar
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    Saleen is correct. Typically what happens in the machine shop:
    1) rods are checked for straightness, length and cracks
    2) Rods are resized (optional, but recommended)
    3) Rods are weight matched to the lightest one, both the large and small ends are done separately. Material is removed from the balancing pad.
    4) Pistons are weight matched to the lightest one. Material is removed from the piston.
    5) Piston rods are matched as well, but seldom off.
    6) Piston and rods with bearings, rings, pins and nuts/bolts are weighted to find the bob weight.
    7) Crank is ground and polished if necessary and any other machining is done such a polishing the counterweights.
    8) Crank is spun balanced with the calculated bob weight bolted to the rod journals. Weight is added or removed from crank counterweights.
    9) Pistons and rods are assembled.

    Sometimes you can get remove the pin from the rod without damaging the piston.

  7. #7
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    That sounds very thorough.

    But I'm not a horsepower man. I just want reliability. I have a 3.9L V6 in my Dodge (same motor minus 2) and I never rev over 4000 RPM.

    So my question to you would be: is this going to make a difference?

    The shop said they would weigh the number-one piston (which I dissassembled) and work out approximate weights for the remaining seven.

    It's a caste Scat crank, original rods, and cast pistons.

    ]Paul

  8. #8
    Member SilverXJ's Avatar
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    A balanced engine helps in the reliability department. And those aren't really horse power tweaks. You can add to the list for that.

    You won't get a perfect balance, but the factory isn't a perfect balance anyhow. They should be able to get you close enough. How did you take the piston off the rod?

  9. #9
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    Quote Originally Posted by SilverXJ View Post
    How did you take the piston off the rod?
    These pistons have a flat shoulder on either side of the hole. I put two machining blocks there. Piston is on it's side now with the pin axis vertical and the machining blocks on the shop press arbor plates.

    Then from the top I made a die on my lathe to fit in the hollow core of the wrist pin.

    I smothered the whole deal in thick gear oil, and pumped the press.

    No scuffing is apparent inside the hole. So I think I am ok.

    silv-o-lite 1278-030


    piston-cut.jpg
    Last edited by paulsheer; 05-03-2015 at 03:26 PM.

  10. #10
    Member SilverXJ's Avatar
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    You should be ok.

  11. #11
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    Update:


    When you press out the pin you slightly crush the wrist pin hole. It's unavoidable because there is a casting imperfection on the far side of the hole inside the piston where the connecting rod eye makes contact when you press.

    You have to sand this down -- it's on the edge of the hole only (on the inside of the piston).

    I did not notice it, and when I got the crank back from balancing i re-heated the eye to find the pin did not go smoothly all the way through both ends.

    I really got into a panic because I thought I would need to redo my piston job all over.

    I pressed it out again and sanded out the small bit of out-of-round.

    Fits back together fine.


    I think you can press out the pins better by make a copper shim to fit between the connecting rod eye and the inside of the piston.

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