The physical properties of objects cannot typically be adjusted post-manufacture to meet specific needs or preferences. While 3D printing offers the potential of design-time customization, manipulating object properties after the object has been printed remains challenging. We present a 3D printable bistable mechanism with an actuation force that can be quickly and repeatedly adjusted after fabrication by up to a factor of 14, enabling both tunable spring-like or lock-like behavior. Our mechanism, printable on commodity low-cost 3D printers, incorporates off-the-shelf elastic threads to maintain robust operation when fabricated at different sizes. We present an evaluation of how manipulating geometrical parameters influences the post-print behavior of our design, and demonstrate its versatility in five functional objects.