HOW TO GET A SCREW HEAD TO SIT FLAT
FOR A SCREW HEAD TO SIT FLAT, the insert needs to be perpendicular. To achieve this on the
boat, first drill a perpendicular hole through a wooden block with the aid of a drill press or
drill guide. You want the hole the same diameter as the bolt or threaded rod you’re using as
an installation tool. Add identical wood blocks to either side of the hole as “feet” to create a
bridge with enough space beneath to accommodate the insert and lock nut. Put the shaft of
your tool up through the bridge before chucking it in the driver. When the bridge sits flat on
the surface, the insert will go in perfectly perpendicular.
head from the bolt and chuck the whole rig
into a power screwdriver. When installing
inserts this way, putting the slot in the hole
first makes for a neater installation, and the
slot helps to cut the threads.
COMPOSITE OR FIBERGLASS
The usual procedure for thick or cored
composite is to epoxy a knurled insert into a
snug-fit hole. An alternative is to tap the hole
in the composite and install an STI, coating
both hole and insert with epoxy. With either
type, the insert should be mounted on a well-greased screw to protect the internal threads
from the epoxy. A bonded insert will be both
stronger and more durable than a sheet-metal
screw driven directly into the fiberglass, but
it is not a suitable substitute for a thru-bolt
with a backing plate.
The well nut is yet another type of
threaded insert – a rubber bushing with a
flange at one end and a nut imbedded in the
other. Tightening the screw compresses the
bushing, causing it to swell. Only as strong
as the rubber, well nuts are light-duty fasteners, but ideal for some marine applications
because they seal, insulate, cushion, and can
be installed without rear access.
To get a full sense of the vast array of available inserts, search the Internet for “screw
thread inserts” and select “images.” And the
next time you think “bigger screw,” consider
a threaded insert instead.
Don Casey has written eight books on boat
repair and maintenance including This Old
Boat, a comprehensive guide to refitting an
older, fiberglass boat.
(could also use a
nutdriver or drill)
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