This is a replacement drive sprocket (the gear that moves the chain) for certain Alpine or Castelgarden brand electric chainsaws. The key specs are a 3/8 inch pitch and 55 teeth. The part number is 656058 and it's made in Italy by Tecnogarden. If your chainsaw's chain isn't moving smoothly or the old sprocket is worn out and damaging the chain links, swapping this in can make it run like new again. Make sure it matches your saw's model before buying, but if it does, it's a straightforward fix that'll extend the life of your whole chainsaw setup. Brand new in its packaging.
