This 1/4x20 hex screw has been machined with a 3/8 inch ball nose end mill to interface with a 3/8 inch ball magnet. The remainder of the allen key hole works great for packing grease into it. Standard automotive grease is required for operation. This hex screw is designed to be glued into the arms (9/32" aluminum tubing, carbon fiber, etc.). The magnet will attract this socket and provide a very solid interface with no backlash. The threads of the screw provide lots of surface area for the glue to bond. Actually screwing this into an arm may be very difficult to get the exact length, and all 6 arms must be exactly the same length for best operation. For instructions on how to make the arms, please see the YouTube video.