This is a pack of ten replacement Picco Boost Wrist Pin Retainer Clips, and are intended for use with the Picco Boost 5TR .21 nitro engine. These clips are used to retain the wrist pin inside the piston, which secures the piston to the connecting rod. If you ever remove the clips during engine maintenance, the clips can become damaged. It is recommended that you replace them with new clips to prevent possible failure caused by used clips.