The Spy Bolt from Shomer-Tec is an updated and improved version of the old Soviet KGB "dead drop" hollow bolt used during the Cold War. It features an O-ring seal for weather-proof storage of small valuables, communications, etc. It has reverse threading on the bolt head for additional security. This new version utilizes a 3/4" diameter bolt providing a larger storage area than the original. Its secret storage compartment has a diameter of .49" and a maximum depth of 2.7". Overall length is 3.43".