Fig. I: To grind a bottom drill, start by grinding the end
of the drill flat and square. Continue by grinding the
back of each cutting lip approximately 20° from flat.
quarter turn, back it out to break the chips and allow
easier tapping. Continue until the tap bottoms out,
then remove the tap and clean the hole.
5. TEST AND TWEAK.
Test the thread engagement between the bolt head
and shank to see if the shank will thread at least
2 threads. If not, it may be necessary to grind off
the first thread of the tap, enabling it to cut down
deeper at full thread diameter as you re-tap the bolt
head. Deburr and clean everything, and your spy
bolt is complete.
Illustration by Julian Honoré/ p4rse.com
Besides being used as a dead drop device, this
hollow bolt can be used to conceal anything small
enough to fit in its cavity. If there’s enough room in
the head, a small O-ring can be used to make the
bolt waterproof. Keep the device protected with a
light coat of oil when it’s not in use. The bolt was
likely plated when new, but machining it exposes
the uncoated steel that will eventually corrode
when exposed to moisture.
Brian Dereu is a self-employed manufacturer who enjoys
gadgets, fishing, and family.
Fig. J: Make a tapered center from a small metal rod.
Fig. K: Tap the hole in the head. Fig. L: Hash marks on
bolt heads indicate the grade of steel. Use Grade 1 or 2.
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
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
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