I worked on my buddy's 440 forged crank to get his reciprocating assembly balanced properly.
It needed to have weight added as it had been balanced for racing previously. To do that, I added a type of heavy steel wool into the hole (the steel wool has a lot of lead in it).
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This gets pounded into the hole, little by little, until it forms a smooth, flat surface (no picture of this - I forgot
). The amount you add needs to be calibrated to the imbalance - you don't want to add way too much or too little. Best is just a "touch" too much so you can final adjust exactly where needed.
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Once the steel wool has been pounded in, a cap is placed in the hole and the center gets pounded to help hold it in (the weight of this cap needs to be accounted for as well).
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Finally, to be sure it never comes out, weld is added to make sure it never moves!
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Next step is to put the assembly back on the balancer and perform (hopefully) final touches to get it balanced.