Right between the painted fins just behind the front hook is normally the balance point. How much depends on what you want it to do. I don't like the factory weighted suicks because there's too much weight in them. They put 5/8 oz in the 9" and I think it performs better with 3/8. You can drill and pour lead but it's kind of a pain. It's easiest just to drill a hold the right size for a 3/8 oz egg sinker and then epoxy it in place.

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By adding only the insert you don't change the action of the unweighted and you can simply screw in a weight as needed.