Garmin, Hummingbird, Lowrance Tamper Resistant Locking Knobs. Included is a set of 2 tamper resistant knobs with M6x1 threaded bolts and rubber washers to make for a secure fit. Due to the shape of the knobs, they’re impossible to loosen by hand when tight. Also included is 1 safety torx bit. Do not tighten using a drill as this could damage your head unit. Please verify the bolt size is correct before purchase. Feel free to reach out for help verifying bolt size.