The Custom Part is a collectible item in Super Smash Bros. for 3DS and Wii U. They're white bags with a golden wrench on the front. Collecting one gives the player a custom special move, a Headgear or Outfit for Mii fighters, or a peice of equipment.
Custom Parts can be obtained in several ways. They appear randomly during normal Smash battles when items are on, they can be found inside the chests in Smash Run, they are somtimes found in the rest area in All Star, Mew can drop them, and they appear on the ground of the Home-Run Contest stadium.
While they don't appear as items, things with the same function as Custom Parts appear in Trophy Rush and Target Blast. In Trophy Rush, Custom Parts make an appearence in the form of Custom Part Blocks. In Target Blast, green targets with a wrench serve as the Custom Part's target blast equivalent.
The white bags with the little wrenches on the front contain custom parts that you can use to give your fighters that little more "oomph". Maybe you'll find a custom special inside for one of the characters, or a hat or outfit for your own Mii Fighter. Either way, open all the treasure chests you see and defeat lots of enemies in Smash Run to get them.
- Villager can store Custom Parts with his Pocket move. This doesn't count as collecting them, but pulling them out does.