The Blade Adapter is used to connect the mower blade to the base of the mower deck. The part is individually sold, silver metal, and measures approximately 2 inches in diameter. Installation requires a Wrench or Socket and is rated as "Easy". If damaged (likely due to fatigue and general wear) there maay be a vibration or rattling sound from it coming loose. Refer to your make, model and diagrams to ensure this is the correct part.