Heat Set Insert Kit: What Is Actually in One and What You Need

A heat set insert kit is a set of brass threaded inserts in multiple sizes — typically M2 through M5 or M3 through M6 — sold in a sorted case. The kit contains the inserts only. The installation tool (a soldering iron or dedicated press), driver tips, and the drilled host part are all sold separately.

In short: a kit is inserts only — no installation tool, no tips, no drill. A typical kit spans M2 to M5 or M3 to M6, with 20 to 50 pieces per size. Installing them needs a temperature-controlled iron with a heat-set tip, or a dedicated press, not a standard craft iron. Pilot hole diameter is set by the insert's outer diameter, not the thread size. Before ordering, confirm insert style (tapered vs. straight), length, and that the supplier provides a drawing with outer dimensions.

What a heat set insert kit contains

The product sold as a 'heat set insert kit' or 'threaded insert assortment' is almost always a collection of brass inserts sorted by thread size and, in some kits, by length. What is physically in the box: brass inserts, typically in metric sizes from M2 to M5 or M2 to M6 — the body is brass, hard enough to accept a steel screw repeatedly without stripping and soft enough to be pushed into thermoplastic with moderate heat. Most kits aimed at FDM 3D printing use inserts with a tapered outer profile and a knurled shank; the taper helps the insert self-centre in the pilot hole, and the knurls bite into the surrounding plastic when the material re-solidifies, providing pull-out and torque resistance. Most mid-range kits also include a compartmented plastic sorting tray or case, though the quality of the case varies and it is not precision hardware. A typical kit of 200 to 300 pieces distributes 20 to 50 inserts per thread size, with larger sizes such as M5 and M6 usually getting fewer pieces per size than smaller ones.

Some kits cover only a single thread size in multiple lengths; these are more useful once you know which size your design needs. Thread standard is nearly always metric (M-series) in kits marketed for 3D printing. UNC or UN assortments exist but are sold separately.

What the kit does not include — and why that matters

The inserts are the least expensive part of the setup. The kit omits an installation tool — no iron, no press — which is the part most buyers underestimate: a standard soldering iron with a pointy tip is not the right tool, because it sits in the insert bore and tilts the insert as it descends. It also omits heat-set tips, the brass or steel adapter nozzles that press on the flat top face of the insert rather than inside the bore; tip sets are sold separately and are usually size-specific, so an M3 tip does not seat an M4 insert cleanly. There is no drill or drill press either — the pilot hole must be drilled before installation, and if the hole is undersized the insert seats crookedly, while if it is oversized the knurls cannot grip. Finally, most kits list no installation temperature: the required temperature depends on the filament material, not the insert, so a separate temperature reference is necessary.

The installation tool: iron vs. dedicated press

Two approaches are in common use. The choice determines how consistent your installations are.

Temperature-controlled soldering iron with heat-set tips

A variable-temperature iron (commonly a Hakko or TS100-class unit) with a tip set matched to your insert sizes is the standard low-volume setup. The tip contacts the insert's top face and pushes it down by hand pressure. Advantages: lower upfront cost, works on a bench with no column or mounting. Disadvantages: the angle and depth depend entirely on the installer. Without a reference surface or depth stop, inserts seating at 2–3 degrees off vertical are common.

Dedicated heat-set press

A mechanical column press with a heated tip holder — such as the P1 Mini Precision Press — keeps the insert perpendicular to the work surface because the tip travels along a fixed vertical column. A depth stop limits how far the insert goes in, so every seat is at the same depth. This matters when producing multiple copies of the same part and needing each insert flush with the surface. The press is the more appropriate tool for any quantity above one-off prototypes.

Lead time, MOQ and pricing for the P1 are available on request. Specifications on this site reflect what has been verified; unverified claims are not listed.

Pilot hole sizing: matching the insert's outer diameter

The pilot hole diameter is determined by the insert's outer diameter (OD), not the thread size. An M3 insert has a 3 mm internal thread but its brass shank is wider — typically around 4.6 mm OD for the tapered style. The hole is drilled to match the shank, not the thread.

Common starting points in PLA and PETG: M2 uses a 3.0–3.2 mm pilot hole (verify against the supplier drawing); M2.5 uses 3.4–3.6 mm and often shares tooling with M3; M3 is the most consistent figure across brands at 4.0 mm; M4 is typically 5.6 mm; and M5 is typically 6.4 mm.

These are starting points only. FDM printed holes come out under the modelled size due to slight over-extrusion on curved walls. Print a test coupon with two or three hole diameters and measure before committing to a final part. Full figures with wall thickness and boss size recommendations are in VERTIPRESS's heat set insert hole size chart.

What to verify before ordering a kit

Not all heat set insert kits are interchangeable. Before ordering, check five things. Insert style: tapered (self-aligning) versus straight-knurl — tapered is the more forgiving profile for hand installation and common in FDM-oriented kits, while straight inserts require the tool to be held more precisely vertical before heat is applied. Insert length: longer inserts resist pull-out better but require deeper bosses in the printed part, while short inserts fit thin walls but provide less thread engagement, and a kit with a single length per thread size may not suit every design. Thread standard: M-series (metric) for most electronics and maker hardware, UNC for anything interoperating with American standard hardware — the two are not interchangeable. Supplier drawing: the only way to confirm the outer diameter, knurl geometry and internal thread tolerance is from a technical drawing; without it, the pilot hole diameter used in your model is a guess, and many low-cost assortment kits do not provide drawings, which is a meaningful risk if you are specifying a part that cannot be easily reprinted. Certifications: some listings claim RoHS compliance, but whether a specific kit meets the standard depends on the actual lot and supplier — this is not a figure that can be taken from a listing page without a test report, so request documentation before ordering if compliance is a requirement.