Stainless Steel Threaded Inserts, SL Type Knurled Insert Nut M2 M3
Why 303 Stainless Steel Instead of Brass
| Property | 303 stainless steel (this page) | Brass (previous page) | Practical difference |
|---|---|---|---|
| Base color | Silver-grey machined steel | Golden brown | Colour matching with steel or nickel hardware |
| Corrosion behaviour | Austenitic stainless with good resistance in indoor and general service | Brass resists rust and keeps lustre | Both are indoor-safe; stainless is the usual pick when the housing is specified as stainless throughout |
| Machinability | 303 is a free-machining grade, chosen for turned parts | Brass is also highly machinable | Both turn cleanly; 303 keeps a steel appearance |
| Thread wear | Steel thread, harder than brass in most comparisons | Brass thread wears faster under repeated torque | Longer service life where screws are reopened often |
| Weight | Higher density than brass | Lighter per part | Matters in weight-sensitive assemblies |
| Cost position | Usually higher material cost than brass | Lower material cost | Drives the choice in cost-sensitive, high-volume molded parts |
SL Type and Double Twill Knurl
The photos show a knurled body with two sets of opposing diagonal ridges, separated by a plain band, plus a short plain lead-in at the top and a flush-style head.
| Design element | What it looks like | Function in the plastic |
|---|---|---|
| Double twill knurl | Two opposed diagonal ridge sets in the mid-body | Grips in both rotational directions, so torque in either direction is resisted |
| Plain band | Smooth ring between the knurl sections | Limits stress concentration and helps the plastic flow evenly |
| Knurl on two levels | Upper and lower knurled sections | Increases the interlock area along the insert height |
| Flush head | Short cylindrical head at the top | Stops the insert at the surface for a flush result |
| Through thread | Internal thread runs the full length | Full engagement depth for the screw |
Dimensions and Published Size Chart
Dimension definitions from the drawing
| Symbol | Label on the image | Meaning |
|---|---|---|
| D | inside diameter | Thread minor diameter, matching the screw size |
| DK | outside diameter | Body diameter across the knurl |
| L | length | Overall insert height |
| K | 槽宽 / slot width | Range shown per size in the chart |
Published size chart (M2–M8)
| Size | Inside diameter D (mm) | Pitch P (mm) | Outside diameter DK (mm) | Slot width K (mm) |
|---|---|---|---|---|
| M2 | 2 | 0.4 | 3.5 | 1.3–3.5 |
| M2.5 | 2.5 | 0.45 | 3.5 | 1.5–4 |
| M3 | 3 | 0.5 | 4.3 | 1.2–6 |
| M4 | 4 | 0.7 | 5.5 | 0.4–6.6 |
| M5 | 5 | 0.8 | 7 | 0.8–2 |
| M6 | 6 | 1 | 8 | 1–2.8 |
| M8 | 8 | 1.25 | 10 | 1.4–2.3 |
Five Installation Methods
The installation diagram shows five routes, all ending in "installed flush with surface".
| Method | Tool shown | How it works | Best fit |
|---|---|---|---|
| Ultrasonic | Ultrasonic horn | High-frequency energy melts the plastic locally around the insert | High-volume production with controlled cycle time |
| Heat | Heated probe | Contact heat softens the boss, then the insert is pressed in | Prototyping, small batches, repair work |
| Press-in | Press ram | Insert pressed into a moulded hole without external heat | Thermoplastics that accept interference fit |
| Expansion press-in | Tapered pilot, closed bottom | Tapered pilot opens the insert so it expands into the hole; the diagram notes 2 full threads of projection to ensure correct expansion | Blind holes and thin walls |
| Self-tapping | Installation tool with tapping head | Insert is screwed onto the tool and driven into a pilot hole, cutting its own thread | Repair, rework and low-volume assembly |
Material and Finish Options
| Option | Availability | Note |
|---|---|---|
| 303 stainless steel, machined | Standard for this page | Silver machined surface, no plating shown |
| Passivation | Available on request | Removes free iron from the surface after machining |
| Other stainless grades | 304, 316L on request | 316L for higher chloride exposure |
| Other finishes | Per project | Confirm appearance sample before production |
Applications
| Application | Why this insert fits |
|---|---|
| Injection-molded housings | Molded-in or post-mold insert gives a serviceable metal thread |
| Electronic enclosures | Stainless body suits clean, rust-free interior hardware |
| Control boxes and panels | Repeated opening during commissioning and service |
| 3D printed prototypes | Heat or ultrasonic setting into printed bosses |
| Automotive interior plastic parts | Small M2 and M3 threads for trim and modules |
| Medical and lab equipment housings | Stainless thread in plastic where cleanliness matters |
| Repair and rework | Self-tapping or expansion type rescues a stripped boss |
Quality Checkpoints
| Checkpoint | What is checked | Method |
|---|---|---|
| Thread | Size, pitch, go/no-go, run-through | Thread gauges and matching screw |
| Inside diameter | D within drawing tolerance | Gauge per batch |
| Outside diameter | DK across the knurl | Micrometer per batch |
| Length | L within drawing tolerance | Height gauge per batch |
| Knurl | Double twill pattern, depth, band position | Visual against a sealed sample |
| Head | Flush head height and squareness | Visual and gauge |
| Surface | Burrs, tool marks, contamination | Visual inspection |
| Material | Grade confirmation | Mill certificate or in-house verification |
Customization — Product and Packaging
| Area | Adjustable | Buyer provides |
|---|---|---|
| Thread size | M2 to M20, including M2.5, M3, M4, M5, M6, M8 | Target size list |
| Length | Insert height L | Drawing with L and tolerance |
| Knurl | Double twill as standard; other patterns on request | Reference photo or drawing |
| Head | Flush head height and diameter | Drawing |
| Material | 303 standard; 304 or 316L on request | Grade and certificate requirement |
| Surface | Bare machined, passivated or plated | Finish specification |
| Packaging | Bag, blister, plastic box, carton | Packing method and quantity per pack |
| Label and box | Logo, size table, barcode, warning text | Artwork and label copy |
| In-box contents | Size mix and quantity per size | Size list and quantity per size |
| Carton | Mark, packing list, carton size | Marking file and shipping data |
FAQ
Q1. What does SL type mean on this insert nut?
SL designates the body style of this SL type knurled insert nut: a flush-style head with a two-level double twill knurl separated by a plain band. The designation is a shape reference, so always confirm the drawing rather than relying on the letters alone.
Q2. Why choose 303 stainless steel over brass?
303 is a free-machining austenitic grade that turns cleanly and keeps a steel appearance, and a steel thread generally wears better than brass under repeated assembly. Brass remains the lower-cost option with a golden finish. Choose by housing material, colour matching and budget.
Q3. Is 303 stainless steel rust proof?
No stainless grade is rust proof. These stainless steel threaded inserts in 303 offer good corrosion resistance in indoor and general service, but prolonged chloride, salt spray or strong acid exposure can still attack them. For those conditions, 316L is the usual upgrade.
Q4. What is the advantage of a double twill knurl?
Two opposed diagonal ridge sets grip in both rotational directions. A single-helix knurl can favour one direction, which matters in brittle or low-modulus plastics where the insert must resist both driving torque and later loosening.
Q5. Which sizes are available?
M2 to M20 can be produced. The published chart covers M2, M2.5, M3, M4, M5, M6 and M8 with inside diameter, pitch, outside diameter and slot width.
Q6. Which installation method should I use?
Ultrasonic for high-volume controlled cycles, heat for prototypes and repair, press-in for suitable thermoplastics, expansion press-in for blind holes and thin walls, and self-tapping for rework. The choice depends on resin, wall thickness and volume.
Q7. What does "installed flush with surface" require?
The boss must be deep enough for the insert length, the hole must match the method, and the head must seat without standing proud. Confirm L against the boss depth before tooling.
Q8. Do you supply hole diameters for each method?
The images show the methods but do not publish hole sizes, because they depend on resin and insert size. Confirm from the drawing and validate on a test boss.
Q9. Can I use these insert nuts for plastic molding directly in the mold?
Molded-in installation is possible with the appropriate boss design and cycle, while post-mold heat or ultrasonic setting is common for lower tooling cost. Both routes use the same insert; the decision is a process one.
Q10. Are these insert nuts for plastic molding suitable for M2 and M3 only?
No. M2 and M3 are the highlighted small sizes, but the range runs to M20. Small threads need tighter hole control and more careful heat input, so validate the process on the smallest size first.
Q11. Can the length and knurl be customized?
Yes. Height, knurl pattern, head style, thread size and surface condition can be adjusted to a drawing or sample.
Q12. Can packaging be customized?
Yes. Bag, blister, plastic box and carton options, plus custom labels, logo, size table and barcode.
Q13. Is a material certificate available?
The images do not show a certificate. If your drawing or quality system requires a mill certificate or inspection report, request it at quotation stage.
Q14. Are the dimensions in the chart exact?
No. The chart notes it is a single batch of manual measurement with errors, subject to the actual product. Treat it as a reference and confirm critical dimensions against a sample or drawing.
Send your thread size, insert length, boss design and preferred installation method to receive a quotation for 303 stainless steel SL type insert nuts, with samples for fit testing and custom labels for your own brand.