My son helped me to discover that the obvious weak point at the shoulder of the shoulder bolt really is too weak. 
So I have added a hole inside the threaded portion of the shoulder bolt into which to screw a real metal screw to add strength.
My son helped me to discover that the obvious weak point at the shoulder of the shoulder bolt really is too weak. So I have added a hole inside the threaded portion of the shoulder bolt into which to screw a real metal screw to add strength.