Major League Baseball uses wooden bats. Aluminum bats are used from Little League to the collegiate level. A corked bat, whether it be wooden or aluminum, is against the rules and unethical in the sport of baseball. If a bat is corked, let it be for personal use and practice. Although it is easy to cork an aluminum bat, it gives the user of the altered bat an unfair advantage over other players.
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Find a bat that you would like to cork. Aluminum bats are not very expensive, especially one that you are going to modify. Remember, after the bat is corked, you can not use it for competitive play.
Use the screw driver to pry open the top of the baseball bat. There is a little seal where you can tell the top is placed and attached. Add a little force and it comes right off. It may be a little difficult, if this is your first time, so use a hammer or another blunt object to pop it off.
Dump out anything found inside the bat. It should be hollow but particles sometime find a way inside.
Place the tennis balls inside the bat. Make sure to fill up the bat. For a modification, try using racquetballs. A racquetball will rattle a little more inside, since they are a little smaller than tennis balls, but the outcome should be equal or greater.
Place the top back on firmly and glue it back on. The glue must be dry before using the bat, so that the top does not pop off.