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Old 10-22-2013, 01:22 PM
1project2many 1project2many is offline
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Join Date: Jan 2013
Location: Lakes Region, NH
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Quote:
when building race motors, we always balanced the assembly as a whole. balanced the small end with the piston, balanced the large end too... not sure why the pistons themselves would all need to be balanced separately...
For future repairs, and / or to allow the balancing operation to progress when all parts are not available. If pistons are balanced to each other and rods are balanced to common weight, then a replacement piston can be installed without re-balancing the the rod/piston assy.

An additional reason to maintain consistent piston weight (and to match the original weight) is to maintain the crankshaft counterweight or "bob weight" effectiveness. The crank essentially unbalanced to counter the force applied as the pistons peak at TDC. Changing piston weight changes the magnitude of force being applied to the crank.

You can (and I do) "home balance" rods by using a fixture that allows fastening two rods to a common pin and allowing the pin to roll on a level surface. The heaviest rod always falls. The greatest challenge was making the fixture consistent when balancing the small end. Turns out it produces better results than the chain fixture above.
http://www.beardmorebros.co.uk/Images/Project%20images/engine/5%20balancing%20rods.jpg
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