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Author Topic:   balancing
posted September 03, 2003 07:03 PM
The machinist will make up "bob weights " which will correspond to the weights of the piston, rod, rings, and wristpin. He will attach these to the rod journals and put the crankshaft into a balancer (either a computerized balancer or a strobe light type) and spin it up. He will then drill holes into the crankshaft and either leave them empty (to remove excess) or fill them with Mallory (to add weight)
The rods will be balanced from end to end and also "weight matched" within just a gram or two of each other. About all that can be done with the pistons is to weigh them and get them all to the same weight.
What I have never understood is why some motors are externally balanced with offest harmonic balancers and counterweighted flywheels. Can someone help me out with that?

posted September 03, 2003 09:26 PM
Generally what you will find with stock production engines that are externally balanced , is that there can not be enough counter weight on the crank to balance internally . This could be for several reasons , such as counter weight to piston clearance (like on a stock 400 sb Chevy) , counter weight to block clearance , extremely heavy bob weight , a combo of the previous three things , etc. You can have externally balanced engines internally balanced , but this is costly due to the extra time spent adding heavy metal , as well as the additional price of the heavy metal .

Regardless of weather or not you have an internal balanced engine , you should always provide your machinist with everything that bolts onto the front and back of your crank (balancer , pulleys , flywheel/flex plate , and pressure plate . Everything should be checked as an assembly .


Hendren Racing Engines
Rutherfordton , NC

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