YOU KNOW THE MOMENT, when you decide to tighten that screw or nut just a little more and, whoosh, the driver suddenly spins. Now what? The smart answer is to install a bigger screw. Sometimes that works. Another ploy is to fill the hole with something — anything from matchsticks to space-age polymer —
then reinstall the same screw. This also works. But a better course, and one too
rarely considered by boat owners, can be to use a threaded insert.
Let’s get up to speed. For 75 years, gearheads and bikers have been repairing stripped
threads with a tightly wound stainless-steel spring called a HeliCoil. The wire of the spring
is diamond shaped, effectively creating threads on both the inside and
outside of the coil. Drill the stripped hole slightly oversize, tap it to
match the outside threads, thread the HeliCoil into the hole (made
easier by a slight reduction in diameter caused by “winding” the coil),
and you have new threads the same size as original – better, really,
because the new ones are wear-resistant stainless steel.
HeliCoil is often used to refer to any insert with threads on both
surfaces. But the correct generic term is screw thread insert (STI). STIs
create solid, near-permanent threads inside a damaged screw hole.
The installation of an STI is exactly like that of any threaded fastener.
You need a pilot hole the root size of the insert; the package the insert
comes in will normally specify the drill size. It is essential to drill the
hole to the correct size. It is also important for the hole to be straight
(see sidebar). The material you’re working with will determine the best
insert and how to install it.
If the insert is being installed in metal, most
will require tapping the hole, although there
are some self-tapping types for soft metals.
The tap is likely to be a special size and
thread intended just for this particular insert
and typically comes with the insert as a kit.
Coil inserts may also require an insertion
tool to tighten the coil.
Knurled inserts are typically pressed into
plastic with the aid of heat. The amateur way
is to thread the insert onto a bolt. Use pliers to hold the bolt with the insert in place
on the hole, applying downward pressure.
Heat the bolt with a soldering iron. The heat
transfers to the insert, which after a minute
or so should begin to sink into the softening
plastic. Continue with downward pressure
until the insert is at the desired depth, then
remove the heat and hold the bolt steady
long enough for the plastic to stiffen.
Inserts intended for installation into wood
can be the barbed type, which are pressed or
hammered into the hole. But as a rule, STIs
will prove more durable. These are typically
spun in, much like a wood screw. Some have
a hex socket and are installed with an Allen
wrench. Slotted inserts can be installed with
a screwdriver, but because the slot is fragile,
the better method is to thread a nut, then
the insert, onto a long bolt. Lock the nut
against the insert, then turn the insert into
the hole with a wrench or socket. Or cut the
PRACTICAL BOATER | DO IT YOURSELF
How to simply create solid, near-permanent threads
inside a damaged screw hole BY DON CASEY
HeliCoils — the
for use in metal.
Have a persistent leak
piece of hardware? A blind
insert – meaning the bottom
is closed –
provide a permanent seal.