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Internally and Externally Balanced Explained in Detail...

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Jeff427, Oct 30, 2000.

  1. Jeff427

    Jeff427 1/2 ton status

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    This question has been asked under different topics, so I thought I would start a thread under this topic, and try to explain it as best I can [​IMG], so here goes;

    Externally balanced motors have weights added to the flywheel and harmonic balancer, to compensate for the crankshaft being out of balance (the counter weights on the crank shaft are not big enough to compensate for the weight of the piston, rod and longer stroke [383, 400sb, 454bb). Internally balanced motors the counter weights are big enough to balance out the weight of the piston, rod and stroke, so weights on the flywheel and weights on the harmonic balancer are not necessary (all other small and big blocks).

    I hope this helps. Any questions, feel free to ask [​IMG].


    Jeff427
    <font color=red>Mud Dog Off Road Club</font color=red>
    http://muddogoffroad.coloradok5.com
     
  2. rich'90

    rich'90 1/2 ton status

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    First question. Why do they drill out all sorts of portions of the crank. You would think that instead of putting the balance weights on the outside they just would not drill as much. PAW drilled so much out of mine that it broke in less than 300 miles. Any ideas?
    Thanks

    Go heavy or Go home[​IMG]
     
  3. FWP

    FWP CRS

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    Great, thank you, that clears up a lot of fuzzy details. But, brings up some more questions. Why have a harmonic balancer at all, if it's not necessary? I've also noted that my 350 motor has some weights on the flex plate??? When you convert a 350 to a 383, using a 400 crank, what components, and shop services, are necessary to balance the motor. Sorry for all the questions, just trying to understand the differences between stock 350 and converted 350 to 383, as I may do it someday.

    <font color=blue>Maverick</font color=blue> <font color=red>CH 4 or 35</font color=red>
     
  4. Jeff427

    Jeff427 1/2 ton status

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    Rich, the reason they still drill the crank is because the weight added to the flywheel and balancer are set amounts, so to bring it into specification, they have to drill the crank. It seems like they could just not drill it but cast cranks are not consistent enough. So to keep the external balance weight consistent, they drill the crank.

    FWP, the harmonic balancer is a necessity to dampen harmonic vibration. The harmonic balancer for externally balanced motors has a large notch in it because the crankshaft is not balanced. The harmonic balancer for internally balanced motors doesn't have the notch because the crank is balanced.
    Flexplates for internally balanced motors have weight added to balance the flexplate independantly from the crank. Flexplates for externally balanced motors have weight added to balance the crank.
    If you want to run a 400 crank in a 350, you have to use a flywheel or flexplate, and a harmonic balancer for a 400. <font color=blue>I think the main journals on the crank have to be turned down in size to fit the 350 block, but I'm not sure, someone else can verify this maybe?</font color=blue> I do know that you need to take the 350 block 0.030 over to get 383ci, with the 400 crank.

    Jeff427
    <font color=red>Mud Dog Off Road Club</font color=red>
    http://muddogoffroad.coloradok5.com
     
  5. rich'90

    rich'90 1/2 ton status

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    Jeff, thanks for the info, that clears it up for me. You are correct, the journals do have to be turned for the crank to fit.

    Go heavy or Go home[​IMG]
     
  6. Nike

    Nike Registered Member

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    I do not know much about turning down the journals to make a 383 out of a 350 block, but as I remember, I did one several years ago and purchased special main bearings. The block was bored .030, and I had to take some cast iron off the inside of the block to get the 400 crank to fit. Rest of the deal was very easy. Thanks, Nike in Mo

    Blue
     
  7. rich'90

    rich'90 1/2 ton status

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    Come to think of it, Nike may be right. I can't remember now which they did. Maybe it can be done both ways, not sure.

    Go heavy or Go home[​IMG]
     
  8. keith

    keith 1/2 ton status

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    In the up coming weeks I will be writing a detailed review on a stroker motor build up. It will be on offroad.com. I ordered my parts and had the block machined already just waiting on the parts. I 'll try to get as technical as I can with out getting over our heads.My build up will be pretty much straight forward. I will include all part numbers and prices. So everyone can see what it really costs to build this motor.
     
  9. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Excellent Keith!! Looking forward to reading it![​IMG][​IMG][​IMG][​IMG]

    Brian
    89KBlazer
     
  10. nyyef

    nyyef 1/2 ton status

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    i built a 383 many years ago, my second build up. you can either way, doing the block or the crank, on mine i did the crank. it was a very good motor. ran strong, many miles and very reliable. reagrding the balancing, at the time didnt know much about it. so i used the 400 flex plate and invested in a BIG fluidamper, worked great (i was lucky)!!

    i do have a question about balancing, does any 400 flexplate and balancer work on any 383 & 400 motor? or do you have to get it balanced for a particular motor each time you do a build up?

    Knife.
    knife.coloradok5.com
     
  11. Jeff427

    Jeff427 1/2 ton status

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    nyyef, yes you can use any 400 SB<font color=red>*</font color=red> flexplate, and harmonic balancer on any 383 or 400 SB<font color=red>*</font color=red>, because the flexplate, and harmonic balancer are set, and any variation in the crank is removed from the counterweights on the crank.

    <font color=red>*</font color=red>The 400 big block is internally balanced so the flexplate/flywheel and harmonic balancer for the 400 big block can NOT be used on the 400 small block.

    Jeff427
    <font color=red>Mud Dog Off Road Club</font color=red>
    http://muddogoffroad.coloradok5.com
     
  12. FWP

    FWP CRS

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    Ah- thank you all, The light bulb has finally come on.

    <font color=blue>Maverick</font color=blue> <font color=red>CH 4 or 35</font color=red>
     

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