Rivet Nuts: Head, Body, Open or Closed End

Contents

1. What a rivet nut is: principle and applications

A rivet nut is a permanent, single-sided threaded fastener: a flanged hollow body with an internal thread in a thin-walled sleeve.

The nut is inserted into a prepared hole, the mandrel of the setting tool is threaded into the back of the nut, and the tool pulls. The thin wall collapses plastically against the far side of the sheet, clamping the panel between flange and bulge. The mandrel is then unscrewed, leaving a permanent internal thread.

The whole operation happens from the front face, with no access needed behind the panel. That is the essential difference from weld nuts and clinch nuts.

Typical applications

• Sheet metal enclosures: server racks, distribution boxes and telecom cabinets in 0.8–2.5 mm sheet that need M4–M8 threads.
• Electric vehicles: battery pack housings, e-drive casings, door trims and dashboard frames, mostly closed-end sealed versions.
• Aluminium extrusions: 6063 profile with 2–3 mm walls where a direct thread strips, so a rivet nut provides the strength.
• Composites and plastics: SMC, GMT and glass-filled nylon cannot be tapped at all; a rivet nut is the only option.
• Tubular structures: square and round tube where the inside is unreachable, so setting must happen from one open end.

2. Rivet nuts versus weld nuts, clinch nuts and flanged threads

There are five common ways to put a thread in thin sheet, and the wrong one strips or scraps the part.

3. Head styles

The head determines how far the fastener stands proud of the panel.

3.1 Flat head

Large flange, head height K ≈ 0.8–1.7 mm. The advantages are a large bearing area and high pull-out strength; the disadvantage is the protrusion, which rules out flush surfaces.

Use inside cabinets, on structural parts, anywhere appearance does not matter.

3.2 Small head

Smaller flange, K of only 0.5–0.75 mm, so the profile after setting is low. Use on visible faces, thin panels and anywhere a cover must sit flat.

3.3 Countersunk head

A 90° cone that sits in a countersink and finishes flush with the panel. Use on decorative panels or where the screw head must be flush.

4. Body styles

The body determines resistance to rotation in the hole, and it is the parameter most often got wrong.

4.1 Knurled body, round hole

Axial knurls against a plain round hole; rotation is resisted by friction.

• Advantage: the hole is a simple round cut, punched or laser cut in one pass, lowest cost.
• Disadvantage: weak torque resistance, worse in soft sheet. On steel at real torque the knurl can spin.
• Use: aluminium, plastics, thin sheet, tightening torque ≤ 2 N·m.

4.2 Semi-hexagonal body

The lower body is knurled and round, the upper section hexagonal, matching a combined round-and-hex hole. The hex section keys into the sheet and resists rotation mechanically.

• Two to three times the torque resistance of a knurled body.
• Requires a shaped hole, laser cut or punched.
• Use: steel sheet, higher tightening torque.

4.3 Full hexagonal body

The whole body is hexagonal in a full hex hole, giving the highest torque resistance — comparable to the nut itself.

• Use: structural parts and higher torque in steel.
• Disadvantage: the hex hole costs more to make and tends to tear in thin sheet.

4.4 Inside and outside hex

Hex outside and hex inside, so the nut can be held from the rear with a wrench. Used for very high torque or where the joint must be tightened from behind.

5. Open end versus closed end

5.1 Open end

The rear of the nut is open, so the thread is visible from behind after setting. Lighter and cheaper. Use where sealing is not required.

5.2 Closed end

The rear is sealed after setting:

• Water and dust cannot pass through the thread into the cavity.
• No debris: an over-long screw cannot pierce a harness or a battery cell.
• Corrosion: the internal thread never sees external moisture.
• Use: battery packs, outdoor cabinets, chassis, medical equipment.

5.3 With sealing ring (EV specification)

An EPDM or silicone ring under the flange (orange or white) is compressed between flange and panel during setting, giving IP67 sealing. This is the standard for battery packs and power electronics housings.

6. Material selection

7. M3–M12 metric table with real grip ranges and pilot holes

Data below is for the flat-head knurled series, in millimetres. Read the panel thickness E first, then the thread.

Closed-end series, typical sizes

8. Tooling and process

8.1 Setting tools

• Hand tool: prototypes, repair, small batches. Short stroke and low force, suitable for M3–M5.
• Pneumatic tool: production lines, 15–25 kN of pull, suitable for M4–M10.
• Hydraulic or servo-electric: automotive lines, with force and stroke monitoring and traceable data.

8.2 Standard sequence

1. Punch or laser cut the pilot hole to the datasheet dimension A.
2. Insert the nut from the front, flange against the panel.
3. Thread the mandrel fully into the rear of the nut.
4. Pull the trigger; the nut collapses against the far face.
5. Reverse the mandrel out.
6. Inspect: flange seated, bulge symmetric, thread undamaged.

9. Decision tree

1. Does it need sealing?
   Yes: closed end; add an EPDM ring for EV battery packs.
   No: open end.
2. What is the panel thickness?
   Look up grip range E and pick the matching length.
3. What tightening torque?
   ≤ 2 N·m in aluminium or plastic: knurled.
   3–8 N·m in steel: semi-hex.
   Above 8 N·m: full hex.
4. Appearance?
   Internal: flat head.
   Visible: small head.
   Flush: countersunk.
5. Environment?
   Indoor: zinc-plated steel.
   Outdoor or damp: 304.
   Coastal or chemical: 316.

10. FAQ

Q1. Rivet nut or clinch nut?

A clinch nut is pressed in with dedicated tooling, needs an accurate hole, and gives the highest torque and pull-out at the lowest unit cost in volume. A rivet nut is set with a hand-held power tool and suits complex parts and prototypes. Above 100,000 standard sheet parts a year, clinch; small batches, complex shapes and extrusions, rivet.

Q2. How much load does an M6 rivet nut take?

A carbon steel flat-head knurled M6 in 1.5 mm aluminium typically pulls out at 3–5 kN, with tightening torque kept under about 4 N·m. A semi-hex body in steel reaches 8–10 kN.

Q3. Can the thread be tapped after setting?

Not advisable. The thread is finished at manufacture, and setting slightly deforms the tail; re-tapping damages it. Replace a damaged nut instead.

Q4. Is a closed-end nut waterproof?

A standard closed end resists splashes but does not guarantee IP67. For IP67 specify the EPDM ring version and keep panel flatness within 0.2 mm.

Q5. Can the same hole be re-set?

No. Once set, the tail is plastically deformed; removing it enlarges and ovalises the hole and a new nut will not grip. Validate settings on scrap first.