Using rivet nuts for threaded fixings

What a rivet nut actually does

A rivet nut — sometimes called a nutsert, blind rivet nut or threaded insert — is a small tubular fixing with a threaded bore through the middle. You push it through a drilled hole in sheet metal, then squeeze it with a setting tool. The body collapses and mushrooms out on the far side, clamping the panel between the flange and the deformed section. What you are left with is a proper, reusable thread in material far too thin to take one on its own.

The big advantage is that you only need access to one side. On a car bulkhead, a trailer side panel, a motorcycle fairing bracket or the inside of a workshop cabinet, being able to work from the outside alone saves an enormous amount of dismantling. The second advantage is serviceability: a self-tapping screw chews up the hole every time it comes out, whereas a rivet nut gives you a bolt fixing you can undo and refit twenty times without the panel suffering.

Choosing the right size and material

Rivet nuts are sized by the thread they carry, and each thread size has a matched hole diameter. Get this wrong and the insert will spin or pull straight through. As a working guide:

• M3 — 5.0 mm hole
• M4 — 6.0 mm hole
• M5 — 7.0 mm hole
• M6 — 9.0 mm hole
• M8 — 11.0 mm hole
• M10 — 13.0 mm hole

Always check the supplier's chart rather than trusting memory, because body diameters vary slightly between manufacturers. You also need to match the grip range — the thickness of material the insert can clamp. A standard M6 rivet nut might cover 0.5–3.0 mm, with a long version for 3.0–5.5 mm. Using a short insert in thick material means it will not collapse properly; using a long one in thin material leaves the body loose and rattling.

On material: aluminium inserts are light and easy to set but have the weakest threads. Steel is the sensible default for automotive and general workshop use. Stainless steel (A2 or A4) is the one for anything exposed to weather, though it needs more effort to set and will gall badly if you run a stainless bolt into it dry — a smear of copper grease on the bolt solves that.

Head styles matter too. A domed (dome head) insert sits proud of the panel. A countersunk version sits flush, which is what you want on a body panel or a floor. Closed-end inserts seal the hole completely, useful on a wheel arch or anywhere water is going to sit. Ribbed or knurled bodies bite into the hole and resist spinning — worth the extra few pence on soft aluminium panels.

The tools you need

A dedicated lever-type hand setter costs very little and comes with interchangeable mandrels and nosepieces to cover M3 to M8 or M10. It is by far the least frustrating option if you plan to fit more than a handful. Buy one with a decent length of handle, because setting a stainless M8 insert takes real effort.

There are also drill-adaptor setters, which thread onto a mandrel and are spun with a cordless drill. They work, but you need a steady hand and a drill with good low-speed control, otherwise you chew the thread.

If you only need one or two, you can set an insert with a bolt, a nut and two thick washers. Thread a bolt of the same size as the insert through a washer, then the nut, then a second washer. Wind the bolt into the insert, hold the bolt still with a spanner on the nut, and tighten the nut down against the panel. It works, but it is fiddly and easy to set the insert crooked.

Installing one properly

• Drill the correct size hole. Use a step drill or a sharp bit and keep it perpendicular — an oval hole will never hold.
• Deburr both sides. A flap wheel or a countersink bit cleans the hole and lets the insert sit flat.
• Check the grip range against the actual panel thickness with a vernier or a rule.
• Thread the insert onto the mandrel and offer it up square. A set square against the panel is worth the two seconds.
• Squeeze steadily until you feel the resistance rise sharply and the tool stops wanting to turn. Reverse the tool off. Do not keep going for extra crush — that is how threads get distorted.
• Run a bolt in by hand to confirm the thread is clean, then tighten to a sensible torque. M5 and M6 inserts are happy around 6–10 Nm in 1.5 mm steel; treat that as a ceiling, not a target.

When things go wrong

Spinning insert. Almost always an oversized hole or a smooth body in soft metal. Step up to a ribbed insert, or fit a large-diameter washer under the flange to spread the load. If it is already in and spinning, drill the head off carefully, knock it through and start again.

Pulled through. Too much squeeze, or a panel that is simply too thin for the load. Use a closed-end or long insert, or add a backing washer on the blind side if you can reach it. Spreading the load across a larger area is the real fix.

Stripped thread. Usually over-torquing a bolt afterwards. Remember the insert is holding sheet metal, not a cylinder head. If you need serious clamping force, that is a job for a welded or captive nut.

Where they earn their keep

Rivet nuts come into their own anywhere a panel has to come off again: seat and harness mounts, undertrays and sump guards, battery boxes, roof rack feet, mudguard stays, fuse panel covers, hinge plates, checker-plate floor sections and trailer side panels. They are also excellent on workshop projects — mounting a vice plate to a thin bench top, fixing drawer runners to a steel cabinet, or making a removable access cover over a pump or wiring loom.

Fit them with a little care, keep an eye on the hole size and grip range, and you will end up with fixings that feel factory. That is a rare and satisfying thing to achieve with a five-minute job and a handful of small metal tubes.