The Right Way to Bolt Into a Carbon Fibre Boat: Threaded Inserts vs. Helicoils
Mounting a Clark pump for the watermaker onto a bulkhead sounds like a small job. Getting the threading right underneath it is where the actual engineering happens.
Why bolt-into-composite matters more than it looks like it should
The plan here is straightforward on the surface: carbon plates bonded to the bulkhead, tapped and fitted with threaded inserts, so the watermaker mounting bracket bolts into those plates rather than punching bolts straight through the bulkhead itself. Hanging hardware off the inside skin, rather than drilling through structure, keeps the bulkhead intact and avoids creating a new water path through it.
Getting there means choosing between three genuinely different ways to put a thread into composite — bare tapped carbon, helicoils, or thread inserts (sometimes sold as thread certs) — and each one earns its place for different reasons.
A quick word on galvanic corrosion first
Mixing stainless steel bolts with carbon fibre plates raises an obvious question: will they react? Yes, to some degree — but manageably. Tef-Gel on the bolt threads isolates the metal from the carbon and controls the reaction, and with windows and penetrations properly sealed, moisture intrusion — the thing that actually drives corrosion — shouldn’t be reaching these fittings at all. Worst case if something does go wrong: light surface rust staining, or in rare cases minor pitting that looks more like localized crevice corrosion than aggressive electrolysis. Stainless bolts through carbon, properly isolated, have a long track record of causing nothing worse than cosmetic staining. Aluminium is a different story entirely — aluminium fittings in direct contact with carbon corrode fast, eaten away in a way stainless simply doesn’t experience.
Method one: bare tapped carbon
The simplest approach — drill a hole, tap a thread directly into solid carbon or glass laminate, thread the bolt straight in. No insert, no special hardware. This still gets used regularly, and it’s not a compromise when the application suits it: for a bolt that’s going in once and staying there — a swim ladder, for instance, bolted on with a little sealant and genuinely never expected to come off except for rare servicing — a bare tapped hole with sufficient thread engagement is completely adequate.
The rule of thumb for how much thread engagement is enough: minimum length equals the bolt’s own diameter. Go deeper — twice the diameter, two and a half times, more — and holding strength increases further. With roughly 20mm of carbon or glass laminate to thread an M6 bolt into (over three times the diameter), a bare tapped hole holds essentially as well as a metal insert would, with none of the added parts or cost.
Method two: helicoils
Helicoils were originally designed for repairing stripped threads in metal — the classic fix when an engine or mechanical component loses its threading and needs a new one wound in. They translate well to composite work, with real advantages and real limitations.
Fitting one means drilling to the correct diameter, tapping a specific oversized thread designed for the helicoil itself, then winding the coil into that tapped hole using a dedicated installation tool — a bar that threads through the coil’s centre, guided in and released once seated. A small tang at the bottom of the coil snaps off cleanly once installed, using a punch tool, leaving a clean internal thread behind.
Two things matter for getting a helicoil right. First, the coil needs a dab of Loctite or epoxy as it goes in, both to stop it unscrewing over time and to help seal the connection. Second, the bolt going in afterward needs Tef-Gel on its threads — not to bond it in, but specifically so it clears the coil’s threads without galling.
Sizing follows the same length-to-diameter logic as bare tapped holes, but described in “D” terms specific to helicoil sizing: a 1D helicoil (length equal to bolt diameter) is the bare minimum, 2D doubles that engagement for real holding strength, and 2.5D — available in some ranges, including certain colour-coded coil types — is close to the strongest option available in helicoil form.
The real weak point of helicoils versus a solid metal insert: because a helicoil is a coiled spring rather than one continuous piece of metal, it doesn’t create a fully homogeneous connection through the laminate the way a solid insert does. It’s also easy to cross-thread while winding one in — jump a thread partway through installation, and extracting a partially-seated, jammed helicoil without damaging the freshly cut hole underneath it becomes a genuinely frustrating repair task in its own right.
Method three: thread inserts
The current preferred method, and the strongest, most reliable way to put a removable thread into carbon. A thread insert is a solid stainless steel sleeve — internally threaded to the bolt size actually needed (M6 in this application), externally threaded larger (M8) with a self-tapping cutting edge that drills and threads its own way into the composite as it’s driven in, no separate tap required in most cases.
The core advantage: because the insert’s outer diameter is larger than the bolt going into it, the effective bearing area against the surrounding carbon increases — a smaller bolt held by a larger-diameter thread, which strengthens the connection to the laminate itself compared to a bolt threaded directly at its own diameter.
Thread inserts are the clear choice for any application where a bolt will be removed and reinserted repeatedly — mast track sections that need unbolting regularly to feed sails on and off being the standout example, where a fitting might see twenty or thirty removal-and-reinstall cycles over its service life. Metal-on-metal threading holds up to that kind of repeated cycling in a way a bolt threaded directly into composite simply won’t, wearing the composite thread down over repeated use.
Where thread inserts get genuinely difficult
Sourcing matters here. Inserts bought from generic overseas suppliers don’t always match standard thread sizing — an M8 external thread that isn’t actually a standard M8 fine pitch creates real problems if a matching tap ever needs sourcing locally, and finding an exact-match fine-pitch tap in a country where that particular standard isn’t common can turn into a longer search than expected.
A specific installation mistake is worth knowing before it happens rather than after: the self-tapping insert needs to be driven far enough that its leading cutting edge fully clears the material and is properly supported the whole way through. Stop short — particularly in genuinely strong, high-quality prepreg laminate — and the leading edge of the insert can get crushed or squeezed inward slightly under resistance, distorting the internal thread just enough that the bolt won’t seat properly afterward. The fix, if it happens, is going back in with a matching tap to clean out and true up the insert’s internal thread after installation — an avoidable extra step if the insert goes in fully seated the first time.
A second real risk: running the installation tool too fast, or without steady, controlled pressure, can strip or gall the insert mid-installation — the thread visibly deforms, gold-coloured galling appears where the metal has worked-hardened and seized, and the insert jams solid, refusing to go either further in or back out. Recovery is possible but not elegant: splitting the stuck insert open with a flat screwdriver, cracking it enough to release its grip on the bolt, then starting again with a fresh insert. One genuine silver lining when this happens: the hole itself is now already pre-tapped from the failed attempt, making the next insert go in considerably easier than the first one did.
The fix that prevents most of these installation problems is simple and consistent: run the installation tool at low speed rather than high, and apply a light coating of Tef-Gel to the insert before driving it in — not so much that it collects carbon dust and drags it back into the hole, just enough on the leading threads to stop the insert galling against the composite as it cuts its own way in.
A simple homemade installation tool
Thread inserts purchased without a proper installation tool aren’t a dead end — a serviceable one is easy enough to build from an M6 bolt and two nuts, locked against each other to control exactly how far the insert travels up the bolt during installation and stop it riding too far before it’s fully seated.
Choosing between the three
None of these three methods is universally correct — the right choice depends entirely on how the fitting will actually be used. A permanent, rarely-touched fitting with enough laminate thickness to get real thread engagement doesn’t need anything beyond a bare tapped hole. A fitting that comes on and off occasionally, where a thread insert genuinely can’t be sourced or fitted, can still work well with a helicoil, understanding its slightly lower holding strength and higher installation risk. And any fitting expected to be removed and refitted repeatedly over years of service — deck hardware, mast track sections, anything genuinely serviceable — deserves a proper thread insert, sized to at least 2D and ideally 2.5D engagement where the laminate allows it.
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