This is a genuine replacement for the line trimmer gearbox on select outdoor power trimmers. The gearbox is the housing that holds the gears responsible for turning the line reel. When the line reel turns, the string cuts grass and weeds. This part is made of durable metal, but it is also exposed to the elements and a lot of constant vibration, so it can become damaged over time and will need to be replaced. If the trimmer head stops spinning or there is a sudden grinding noise, it could indicate that this part needs replaced. It's important to use an OEM replacement for best results. This unit is sold as a complete assembly and includes the mounting hardware, three screws, head assembly, and the cap. A hex screwdriver, socket or wrench set, and Phillips screwdriver will be necessary to replace this part. Be sure to allow the unit to cool and wear gloves whenever performing repairs.This genuine OEM line trimmer gearbox is manufactured by MTD and compatible with machines from Craftsman.