This is a genuine OEM sourced component which is specially designed for use with Troy-Bilt string trimmers. It is a high-quality replacement for a missing or a damaged cap that was installed on a new unit. This is the part that helps to secure and protect the spool. Refer to the appropriate diagrams of your model for the exact location an...Read mored the correct installation of this part. Keep in mind this high-quality cap is made of durable plastic and is sold individually. Replacing it does not require any tools, simply press on two tabs of the cap to secure it into the trimmer head.