Installing Threaded Inserts
By Don Casey
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 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.
— Published: April/May 2014
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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.