---
title: "Threaded Inserts: Types, Sizing, Callouts and Quoting"
description: "When a threaded insert or clinch nut beats a tapped hole, how each type changes the hole, how to call them out, and how to add them to an RFQ correctly."
canonical: https://openspindle.com/blog/threaded-inserts
author: "Tom"
datePublished: 2026-09-16T00:00:00.000Z
dateModified: 2026-09-16T00:00:00.000Z
category: "Process & DFM"
---

# Threaded Inserts: Types, Sizing, Callouts and Quoting

A threaded insert puts a stronger or more durable thread into a part than the parent material can hold. Helical coil and key-locking inserts suit soft metals and repairs, self-clinching nuts and studs suit thin sheet metal, and heat-set inserts suit plastics and 3D prints. Each needs its own hole size and a decision about finishing order. In an OpenSpindle RFQ, tick Inserts under Secondary operations, enter the total count, and list each insert's part number, quantity and location in the part Note or on a drawing.

## When an insert beats a tapped thread

A tapped hole is the cheapest way to put a thread in a part, and for most holes in steel, stainless or aluminum that is assembled a few times it is the right answer. An insert earns its extra cost in a handful of situations.

**Soft or weak parent material.** Aluminum, magnesium, zinc castings and plastics strip at lower loads than the steel screw going into them. An insert spreads the load over a larger diameter of parent material, so the joint can take more torque before the thread pulls out.

**Repeated assembly.** A thread cut in aluminum wears every time a steel screw goes in and out. Service panels, test fixtures and anything a technician opens regularly are good candidates for a hardened or stainless thread that does not wear the same way.

**Repair.** A stripped thread in an expensive part can often be drilled out, retapped oversize and fitted with a helical coil or key-locking insert that restores the original thread size.

**Material too thin to hold a thread.** Sheet metal thinner than a few thread pitches has almost nothing to engage. Self-clinching nuts, rivet nuts and weld nuts give thin sheet a full-length thread.

**Plastics and 3D prints.** Threads cut or printed directly in plastic tolerate only a few assembly cycles and can creep under load. Heat-set and press-in brass inserts are the usual fix.

If none of these apply, a tapped hole with enough engagement is usually fine. The tapped holes guide covers how deep that needs to be.

## Insert and pressed-in hardware types compared

The names get used loosely, so it helps to be precise in an RFQ. A dowel pin is not threaded at all, but it shows up in the same secondary operation because it is also hardware pressed into a machined hole.

| Type | Parent material | How it is installed | Strength and reuse | Cost and notes |
| --- | --- | --- | --- | --- |
| Helical coil insert | Aluminum, magnesium, castings, some plastics; also thread repair | Hole drilled and tapped with an oversize STI tap, coil wound in with an insertion tool, tang removed if required | Good load spreading, wear resistant stainless thread, free-running or screw-locking versions | Low hardware cost, needs the STI tap, available in several lengths |
| Key-locking insert | Aluminum, castings, steel; high load and repair | Hole tapped for the insert's outer thread, insert threaded in, keys driven down to lock it | Very high pull-out and torque-out, replaceable | Higher unit cost than a coil, needs a larger hole and more wall around it |
| Self-clinching nut, stud or standoff | Sheet metal, commonly aluminum and steel | Pressed with a steady squeeze into a punched or drilled hole so the panel flows into the fastener's undercut | Strong push-out and torque-out in thin sheet, reusable thread | Needs a press, a minimum sheet thickness and edge distance, and a harder fastener than the panel |
| Rivet nut (blind rivet nut) | Thin sheet, tube, extrusions, parts with access from one side only | Set from one side with a tool that collapses the body behind the sheet | Moderate strength, can spin in the hole under torque unless a knurled or hex body is used | Low tooling cost, suits closed sections and assemblies |
| Heat-set insert | Thermoplastics, including FDM and SLS prints and molded parts | Heated and pressed into a slightly undersized hole so the plastic melts and flows into the knurls | Good torque-out and many reuse cycles compared with threads in plastic | Cheap brass hardware, simple install, hole size from the insert maker |
| Press-in or ultrasonic insert | Thermoplastics, molded parts | Pressed cold, or driven with ultrasonic energy, into a molded or drilled hole | Lower retention than heat-set for cold press, similar for ultrasonic | Fast in production, sensitive to hole size |
| Dowel pin | Metals and some rigid plastics | Pressed into an accurately sized, often reamed, hole | Not a fastener: locates parts precisely, usually pressed once | Low hardware cost, hole accuracy drives the price |

## How an insert changes the hole

The single most common mistake with inserts is modeling the hole for the screw instead of the insert. An insert almost always needs a larger or differently sized hole than a tapped thread of the same size, and the exact size comes from the insert manufacturer's installation data, not from a generic tap drill chart.

**Helical coil inserts** are installed in a hole tapped with an STI (screw thread insert) tap, which is oversize compared with a standard tap of the same nominal size. The drill under it is larger too. Coils are sold in lengths expressed as multiples of the nominal diameter, commonly 1D, 1.5D, 2D, 2.5D and 3D, so an M6 insert at 2D is about 12 mm long. The tapped depth has to be at least the installed insert length, plus room for the tang in a blind hole if a tanged insert is used. Tanged inserts need the tang broken off and removed after install in most cases; tangless versions avoid that step.

**Key-locking inserts** need a hole tapped for the insert's external thread, which is noticeably larger than the screw, and often a small counterbore so the keys sit flush. They also need more wall thickness around the hole than a coil does.

**Heat-set and press-in inserts** go into a plain hole, usually slightly smaller than the insert's knurled outer diameter and sometimes tapered to match the insert. Leave the hole a little deeper than the insert so displaced plastic has somewhere to go.

**Dowel pins** go into holes sized for the fit you want. A press fit in one part and a close slip fit in the mating part is the common arrangement, and the reamed or bored hole size is set from the pin's actual diameter tolerance. Blind dowel holes benefit from a way for trapped air to escape, or from a pull-out style pin if the pin may need removing. The engineering fits chart covers the hole and shaft tolerance combinations.

| Hardware | What sets the hole | Depth to allow | Easy mistake |
| --- | --- | --- | --- |
| Helical coil insert | STI drill and STI tap for the thread size | Insert length (1D, 1.5D, 2D and so on) plus tang room in blind holes | Modeling the standard tap drill, so the hole is too small |
| Key-locking insert | Tap for the insert's outer thread, plus counterbore where specified | Insert length plus a small allowance | Too little wall or edge distance for the larger hole |
| Self-clinching nut or stud | Catalog hole diameter, usually with a tight tolerance | Sheet thickness at or above the catalog minimum | Hole too big, or sheet too thin for the part number |
| Rivet nut | Catalog hole diameter for the body, round or hex | Grip range must match the sheet thickness | Wrong grip range, so the nut does not set properly |
| Heat-set insert | Insert maker's recommended hole, often tapered | Insert length plus a little extra | Printing or molding the hole at the thread size |
| Dowel pin | Reamed or bored to the press or slip fit | Pin engagement plus air escape in blind holes | Specifying a drilled hole for a precision location |

## Self-clinching hardware in sheet metal

PEM (or equivalent) self-clinching fasteners are the standard way to add threads, studs and standoffs to sheet metal. They work because the panel material cold flows into a groove on the fastener as it is squeezed in. That mechanism sets a few rules that are worth designing around before the flat pattern is released.

**The panel has to be softer than the fastener.** The fastener must displace the panel, not the other way around. Standard 300-series stainless clinch hardware is widely used in aluminum and in mild or cold rolled steel, within the panel hardness limits the manufacturer lists. Stainless sheet is usually too hard for that hardware; stainless panels generally call for hardware made for the purpose, typically hardened 400-series or precipitation-hardened stainless types. Check the catalog for the exact panel hardness limit before specifying.

**Each part number has a minimum sheet thickness.** Below it, there is not enough material to fill the undercut and the fastener will push out or spin. The minimum is listed per part number and varies by thread size and fastener style.

**The hole size is specific.** Clinch hardware goes in a punched or drilled hole of a stated diameter, with a tight tolerance. An oversize hole, or a heavy chamfer on the install side, reduces the material available to clinch.

**Keep the catalog minimum centerline-to-edge distance.** Too close to an edge and the displaced material bulges the edge instead of flowing into the fastener. The same thinking applies near bends and formed features: hardware close to a bend line can distort during forming, or may leave no room for the press tooling after forming.

**Choose which side it goes in from.** Clinch nuts and studs are installed from one side, with the head or shank flush on the other. The drawing needs to say which face the fastener is pressed from, or every one can end up on the wrong face.

| Panel material | Typical clinch hardware choice | Notes |
| --- | --- | --- |
| Aluminum sheet (for example 5052) | Stainless (300-series) or steel hardware | Common and well proven; aluminum hardware is sometimes used for softer aluminum panels |
| Mild or cold rolled steel | Steel or 300-series stainless hardware | Check the panel hardness limit for the chosen hardware |
| Stainless steel sheet | Hardware designed for stainless panels, typically hardened 400-series or precipitation-hardened types | Standard 300-series clinch hardware is generally not suitable |
| Pre-plated or galvanized steel | Steel or stainless hardware per catalog | Coating and panel hardness both affect the clinch; confirm with the hardware data |

## Install order and finishing

Inserts and coatings interact, so the order of operations belongs on the drawing or in the note. There is no single right sequence for every part, but these are the common rules of thumb.

**Helical coil and key-locking inserts** are usually installed after anodizing, plating or painting. The oversize thread is tapped before finishing, then the insert goes in afterward so the coating does not cover the insert thread. Coating builds up in the STI thread as well, so for thick coatings the shop may mask the holes or chase the threads before install.

**Self-clinching hardware** is commonly installed after plating or anodizing the panel, which avoids plating the fastener threads and trapping plating chemistry around the fastener. Powder coat and paint are handled both ways: install first and mask the threads, or finish first and keep coating out of the holes so the hole size and clinch are not affected. If your part is powder coated, say which you want or ask the shop to advise.

**Heat-set inserts** go in after printing or molding and before any painting or coating of the plastic part.

**Dowel pins** are usually pressed after finishing. Anodize and plating change the hole diameter, so a press fit sized for bare metal can become too tight after coating unless the hole is masked or sized to allow for it.

If you do not specify, the shop will pick a sequence that suits its process. That is often fine, but it is worth stating when threads must gauge after finishing or when coating inside a hole matters. The anodizing vs powder coating guide covers how much each finish builds.

## How to call out inserts on a drawing

A complete insert callout answers four questions: what hardware, how many, where, and anything about orientation or length that the part number does not settle. The manufacturer part number does most of the work, because it fixes the thread, the material, the length and the required hole. Use the manufacturer part number, such as a clinch nut designation from the hardware catalog, or write "or equivalent" if the shop may substitute.

For threaded inserts, give the installed length as a multiple of diameter or in millimeters or inches, and say whether it is free-running or screw-locking. For clinch hardware, name the side it is installed from. For dowel pins, give the diameter, length, material and how far the pin should stand proud of the surface. A leader to each location, or a hole table with a column for hardware, keeps the count and the positions in one place. The thread callout guide covers the thread designation itself.

| Example callout | What it tells the shop |
| --- | --- |
| 4X HELICAL COIL INSERT M5x0.8, STAINLESS, FREE-RUNNING, 1.5D, INSTALL AFTER ANODIZE | Thread, length, style and sequence; the shop sizes the STI hole to suit |
| 6X M4 SELF-CLINCHING NUT (MFR P/N OR EQUIVALENT), INSTALL FROM SIDE A | Hardware and the face it is pressed from; side A is marked on the view |
| 2X KEY-LOCKING INSERT 1/4-20, STAINLESS, PER MFR INSTALL DATA | Insert type and thread, with hole size taken from the maker's data |
| 3X HEAT-SET INSERT M3, BRASS, 5.7 LONG | Thread, material and length for a plastic part; hole per insert maker |
| 2X DOWEL PIN 5 DIA x 16, HARDENED STEEL, PRESS FIT, 6 PROUD OF FACE B | Pin size, material, fit and protrusion height |

## What drives the cost of inserts

Insert hardware is often inexpensive. The installation is where most of the price sits, and it scales with count and variety.

**Count.** Most inserts are installed one at a time, so price rises roughly with the number of pieces. That is why the total count matters to the quote.

**Variety.** Every different insert type, size or length is another tool, another setup on the press and another bin to manage. Standardizing on one or two sizes per part saves more than shaving hardware cost.

**Insert type.** Key-locking inserts cost more per piece and take longer to install than helical coils. Clinch hardware is fast once a press is set up, but special styles such as long standoffs or floating nuts usually cost noticeably more than a plain nut.

**Hardware availability.** Common sizes in stainless are usually on the shelf. Uncommon lengths, materials or proprietary designs can add lead time, and a specific part number with no equivalent allowed limits the shop's sourcing.

**Access and sequence.** Hardware near a flange, inside a deep pocket or on a curved face may need special tooling or an extra step. Installing after finishing means the part is handled again after it comes back from the finisher.

**Hole accuracy.** Dowel pin holes that must be reamed or bored to a precision fit take longer than drilled holes, and the positional tolerance between two pins can drive the setup.

## Adding inserts to your OpenSpindle RFQ

The quote form has a dedicated checkbox for inserts, separate from the one for tapped holes. Ticking the right one, with the right count, is what lets a shop price the hardware and the install time instead of adding it as a change later.

**Step 1: Upload the model and set the Process.** Add the CAD file and check the Process on each part. Inserts appear under Secondary operations for CNC, Sheet Cutting, Sheet Metal, Die Casting and Metal Stamping. The form suggests a process from the file type, so make sure a bent part with clinch hardware is set to Sheet Metal.

**Step 2: Tick Inserts.** Open Advanced options on the part and tick Inserts under Secondary operations. Enter the total number of inserts, clinch fasteners, rivet nuts and dowel pins in Total count on this part, counting every piece rather than every type.

**Step 3: Tick Threads / Tapped Holes separately if the part also has direct threads.** Holes that get a plain tapped thread belong there with their own count. Holes that receive a helical or key-locking insert are counted under Inserts, and it helps to say so in the note so nobody counts them twice. The tapped holes guide covers what to write for those.

**Step 4: Write each insert in the Note.** For every insert type, give the part number or a description, the quantity, the location, and the details the part number does not settle: installed length, free-running or screw-locking, the install side for clinch hardware, pin protrusion, and whether to install before or after finishing. If you are happy with an equivalent, say so.

**Step 5: Attach a drawing.** Under Reference Files & Technical Drawings, attach a PDF drawing with the callouts, or a marked-up screenshot with each insert location circled and labeled.

The Inserts help text states the defaults the shop will use if you give no details: helicoils default to stainless steel 1.0×D, and dowel pins to stainless, full depth. Those are reasonable fallbacks, but they are often not what a design needs. A 1.0×D coil is the shortest common length, and in aluminum or magnesium a 1.5D or 2D insert is a frequent choice for more pull-out strength. The default says nothing about free-running or screw-locking, so name it if vibration matters. Stainless dowel pins resist corrosion but are softer than hardened steel pins, which matters for precision location that sees wear. Full depth reads as the pin pressed to the bottom of the hole; if the pin must stand proud to locate a mating part, give the protrusion height. Clinch hardware, rivet nuts and heat-set inserts have no stated default, so a part number or description is needed for those.

For 3D printed and injection molded parts, the process options do not include Secondary operations, so describe heat-set or press-in inserts in the part Note and on the drawing instead.

| What to include | Example note | Why it matters to the quote |
| --- | --- | --- |
| Insert type and part number | M4 self-clinching nut, MFR P/N or equivalent | Sets hardware cost, hole size and tooling |
| Count per type | 6X clinch nut, 2X dowel pin | Install is priced per piece and per type |
| Location | Bosses on page 2 of the drawing | Lets the shop check access and setups |
| Length or style | Helical insert M5, 2D, screw-locking | Overrides the 1.0×D default |
| Install side (clinch hardware) | Nuts pressed from the inside face | Avoids hardware installed upside down |
| Pin material and protrusion | Hardened steel, 6 mm proud | Overrides the stainless, full depth default |
| Sequence with finishing | Install after anodize | Keeps coating out of the insert thread |
| Who supplies hardware | Shop to supply, or customer furnished | Changes the hardware line and lead time |

## A complete example

Here is a part Note for a CNC machined 6061-T6 aluminum housing, clear anodized, that a shop could quote without a follow-up question:

8X helical coil insert M4x0.7, stainless, free-running, 2D (8 mm long), in the cover bosses on the top face, install after anodize. 2X dowel pin 4 mm dia x 12 mm, hardened steel, press fit, 5 mm proud of the top face, holes sized for press fit after anodize. 4X M5x0.8-6H THRU tapped holes for the mounting feet, no inserts. Shop to supply all hardware, equivalents OK. Drawing attached, insert and pin locations on page 2.

On that part, Inserts is ticked with a count of 10 (eight coils and two pins), and Threads / Tapped Holes is ticked separately with a count of 4.

For a sheet metal cover in 1.5 mm 5052-H32 aluminum, powder coated, the same idea looks like this:

4X M3 self-clinching nut, stainless, PEM or equivalent, pressed from the inside face (marked on drawing), catalog hole size. 2X M4 self-clinching stud, 10 mm long, stainless, pressed from the inside face. Install hardware before powder coat and mask threads, or advise if you prefer a different sequence.

That part uses Process Sheet Metal, Inserts ticked, and a count of 6. If you are not sure which insert suits a joint, write the load and how often it will be assembled in the note, and ask the shop to recommend one.

## Related reading

- [Tapped holes: design, callouts and quoting](https://openspindle.com/blog/tapped-holes.md)
- [Thread callout guide](https://openspindle.com/reference/thread-callout-guide.md)
- [Tap drill and thread chart](https://openspindle.com/reference/tap-drill-thread-chart.md)
- [Engineering fits chart](https://openspindle.com/reference/engineering-fits-chart.md)
- [Sheet metal fabrication capabilities](https://openspindle.com/capabilities/sheet-metal-fabrication.md)
- [Sheet metal gauge chart](https://openspindle.com/reference/sheet-metal-gauge-chart.md)
- [Anodizing vs powder coating](https://openspindle.com/blog/anodizing-vs-powder-coating.md)
- [How to write an RFQ](https://openspindle.com/blog/how-to-write-an-rfq.md)

## Frequently asked questions

### When should I use a helical coil insert instead of tapping the part directly?

Consider one when the parent material is soft, such as aluminum, magnesium or a casting, when the screw will be removed and reinstalled many times, or when you are repairing a stripped thread. For holes assembled once or twice in a material with enough thread engagement, a direct tapped thread is usually cheaper and adequate.

### What length helical coil insert should I specify?

Coil inserts come in lengths expressed as multiples of the nominal diameter, commonly 1D, 1.5D, 2D, 2.5D and 3D. 1D is the shortest and is often enough in steel. In aluminum and other soft alloys, 1.5D or 2D is a common choice for more pull-out strength. Confirm against the load and the available depth in the part.

### Can self-clinching nuts be installed in stainless steel sheet?

Yes, but usually not with standard 300-series stainless clinch hardware, because the fastener needs to be harder than the panel. Stainless sheet generally calls for hardware designed for stainless panels, typically hardened 400-series or precipitation-hardened types. Check the manufacturer's panel hardness limit and minimum sheet thickness for the part number.

### Should inserts go in before or after anodizing or plating?

Helical coil and key-locking inserts, and self-clinching hardware in plated or anodized panels, are commonly installed after finishing so the coating does not cover the insert thread. Powder coated parts are handled both ways, often with masking. State the sequence you want on the drawing or in the note, or ask the shop to advise.

### Are heat-set inserts better than threads in a 3D printed part?

For parts assembled more than a few times, generally yes. Threads printed or tapped in plastic wear quickly and can creep under load, while a brass heat-set insert gives a metal thread with good torque-out. The hole needs to be sized to the insert maker's recommendation rather than the thread size.

### How do I add inserts to an OpenSpindle quote request?

On each part, open Advanced options, tick Inserts under Secondary operations and enter the total number of pieces in Total count on this part. List each insert's part number, quantity, location, length and install side in the part Note, and attach a drawing. If you give no details, helicoils default to stainless 1.0×D and dowel pins to stainless, full depth. Tick Threads / Tapped Holes separately for any direct tapped threads.