When spies need to pass off a note or microfilm without meeting in person, they sometimes use a “dead drop” device. An effective dead drop device looks common enough to blend in without causing suspicion under the casual glances of passersby. This bolt is one such item.
It’s made from an ordinary steel bolt that’s hollowed out and fitted with a threaded, removable head. If made to the correct size and properly painted, it could replace a solid bolt on a fence, bridge, or similar structure. (Of course, we don’t advocate that you remove a real bolt from a bridge! The device described here should be considered a desktop curiosity only.)
A Bit About Bolts
The bolt for this project can be of many sizes. For ease of machining, it should be SAE Grade 1 or 2 steel; both are low-carbon and not heat-treated.
The head of the bolt tells the grade by a series of radial hash marks. No hash marks at all indicates the lowest grade, and therefore the softest and easiest to machine. Select a bolt that has a portion of its shank unthreaded (a long length-to-diameter ratio) which will leave a section available for the new threads.
For this project, I chose a ½-13×2" bolt. In bolt designations, ½-13 means ½" diameter, and 13 threads per inch. The length of a bolt is measured by its shank, not including the head.