Secures the shifter boot on newer Boss 302 Mustangs™ with narrow boot opening when installing a new aftermarket "ball style" shift knob to the factory shifter. Also serves and the jam nut for the shift knob. 12mm x 1.25 thread size. Will also fit Mustangs™ with 401A interior and factory "ball style" shift knob. Please verify your shifter thread size prior to ordering.