My 904 pulled from a 318 question

j par said

It seems like there would be a way of doing that also just like what's done on the torque converter? Like where to space weight exactly to do it, just like drilling a flywheel?

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they didn't "weld a certain amount of weight" and call it balanced. They welded a plate of steel, then they spun it maybe 200 rpm's and balanced with a "light meter". Then they slightly removed weight using a drill. Kind of like tuning a guitar. A lot more precise than welding a certain hunk of steel in a spot.