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  1. #1
    PWCToday.com Is My Home Away From Home SeaDooSource's Avatar
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    2 stroke crank balance questions...

    I have a question that has been on my mind for a while now. It concerns the balance of a sea doo 2 stroke crank (any model). I assume from the factory they're designed to be balanced (or close) with oem parts. So if you add aftermarket pistons and/or rods that may be of a different weight than oem, wouldn't that throw off the balance? And if that's the case, how would you correct? I would guess you assemble the crank without the rods then balance it using bob weights the same way you do an automotive crank? Then disassemble and reassemble with the rods. I'm used to 4 stroke race auto engines so need a little enlightenment on 2 stroke design!
    Last edited by SeaDooSource; 01-16-2007 at 03:53 PM.
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  2. #2
    Tech Guru PWCToday.com Is My Home Away From Home Watercraft Magic's Avatar
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    Re: 2 stroke crank balance questions...

    Well, for starters, balancing any two clyinder upright engine is a crap shoot at best to get them even close to being "balanced" as in what would be normal for a V motor.
    Aftermaket rods and pistons, even very big oversized pistons don't really make a big difference in the balance of these two clyinder 180 degree two strokes, as long as you do not add or take away stroke from the crankshaft.

    I think if you want to do the best you can with a stock stroke motor, you balance the magneto flywheel. You would not beleve how out of balance these are. If you remove the magneto cover and run the motor with a strobe light shineing on the magneto flywheel, you will run for cover. They wobble all over the place. It is downright scary, and no wonder the front main bearings are always rattling after a few hours on the motors.

    It is one of the first things we do when building racing engines for ourselves, and customers who want it done.

    When building strokers, we often add heavy metal to the balancing flywheels on the crankshafts, or use custom made crankshaft flywheels ( aka, porkchops) to help smooth out the motors' balancing.
    Last edited by Watercraft Magic; 01-16-2007 at 04:07 PM.
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  3. #3
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    Re: 2 stroke crank balance questions...

    Quote Originally Posted by Watercraft Magic View Post
    I think if you want to do the best you can with a stock stroke motor, you balance the magneto flywheel. You would not beleve how out of balance these are. If you remove the magneto cover and run the motor with a strobe light shineing on the magneto flywheel, you will run for cover. They wobble all over the place. It is downright scary, and no wonder the front main bearings are always rattling after a few hours on the motors.

    It is one of the first things we do when building racing engines for ourselves, and customers who want it done.
    How is this done?

  4. #4
    Top Dog Cleatusjo's Avatar
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    Re: 2 stroke crank balance questions...

    I also put my OEM flywheel on a rotory table and cut concentric bores into the front cover to knock a pound or 2 off the crank. The table assures your concentric to the crank with your cutouts. If you could make a mandrill for an engine lathe, and the runout isnt crazy , you could take a skim cut across the cover to true it up.

    What is the cost to balance a flywheel ?
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  5. #5
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    Re: 2 stroke crank balance questions...

    Quote Originally Posted by Cleatusjo View Post
    What is the cost to balance a flywheel ?
    I don't know. Maybe someone will chime in. If the flywheels are way out of balance it would be a big advantage to have it balanced.
    Engine would last longer.

  6. #6
    resident guru hafweigthr's Avatar
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    Re: 2 stroke crank balance questions...

    I am rebuilding several 787s and usually use the old flywheels unless they are really chewed up from the starter. I too thought that these things must really get out off balance since they usually have gears chewed up some on one side or the other.

    Some motors and skis (of the same model)are significantly different from one another. I have rebuilt at least a dozen 787-GTXs and they all are different. When you think about it there are so many variables that can be different besides having an imbalanced crankshaft.

    And yes a better built engine with a nearly perfectly balanced crank would last longer. A factory built motor should last much longer than a rebuild shop motor given the methods used and the quality control required.
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  7. #7
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    Re: 2 stroke crank balance questions...

    Good comments, So where (and how) do we get our flywheels and our pto end balanced

  8. #8
    resident guru hafweigthr's Avatar
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    Re: 2 stroke crank balance questions...

    You could probably make a hand rig to balance by cutting off a old crank(flywheel end) and using it with a center hole to make a simple single point balancer for them.

    Balance could be adjusted by fine tuning the teeth on the flywheel(OD or backside) by cutting on a lathe where the teeth wear would be. I would be leery of stick on balance weights @ 7000RPM.
    It also takes very little on a tapered surface to throw off the mounting and balance. It is critical to make sure the tapered surface on the flywheel and the engagement surface on the end of the crank are free of burrs, etc. - The key being fully seated can also affect this as well.

    Maybe the race engine builders could chime in but that would probably give away their secrets.
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