Self-Threading Screws, Thread Inserts and Hole Design: Torsional Strength Reference

Torsional Strength Reference: Inch BF/BT Screws

Per ANSI B18.6.4-1981 (R1991), minimum torsional strength requirements (in pound-inches) apply to tapping screws. Types AB, B, BF, BP, and BT share the same torsional strength column:

Use torsional strength to validate your drive tool torque setting. Set your power driver clutch well below these values — typically at 60–70% — to account for installation variability and prevent cam-out or drive recess damage.

How to Correctly Designate a Metric BF or BT Screw

Per ANSI/ASME B18.6.5M-1986, the method of designation follows this sequence:

[Nominal Size] × [Thread Pitch] × [Nominal Length] [Type] [Head Style] [Driving Provision] [Material] [Finish]

Worked Examples

4.2 × 1.4 × 13   Type BF, Type 1 Cross Recessed Oval Countersunk Head Tapping Screw, Steel, Chromium Plated

6.3 × 1.8 × 25   Type BT, Slotted Pan Head Tapping Screw, Corrosion Resistant Steel

2.9 × 1 × 10     Type BF, Hex Flange Head Tapping Screw, Steel, Zinc Plated

Designation Breakdown

Never abbreviate or reorder this designation when creating purchase orders or engineering drawings. Ambiguous callouts are the single most common source of incorrect fasteners being supplied from stock.

The Universal Takeaway: The Decision Framework

Use the following decision tree every time you are specifying a self-threading screw for a non-metal or cast metal application:

START: What is your substrate material?
│
├── RESILIENT / DUCTILE PLASTIC (polyethylene, polypropylene, nylon, TPE)?
│   └── Thread-forming screws acceptable → Types AB, B, or BP
│
├── RIGID / BRITTLE PLASTIC (ABS, polycarbonate, acrylic, phenol formaldehyde,
│   cellulose acetate, styrene resin)?
│   └── Thread-CUTTING screws required → Types BF or BT
│       │
│       ├── Softer brittle plastic or asbestos composition?
│       │   └── Consider Type BF (gentler cutting geometry)
│       │
│       └── Harder brittle plastic or tight torque control needed?
│           └── Prefer Type BT (more aggressive 90–95° flute geometry)
│
├── DIE CAST ZINC or ALUMINUM?
│   └── Thread-cutting → Types BF, BT, D, F, or T (BF/BT preferred for softer alloys)
│
└── STEEL SHEET or STRUCTURAL STEEL?
    └── Types D, F, G, or T — machine screw thread series

Quick Reference: BF/BT at a Glance

Master Formula Card

For those engineering boss geometry for plastic assemblies, three relationships govern whether the assembly will succeed:

Effective Thread Engagement:

$$L_e = L_{screw} - L_{taper}$$

Where:

• $L_e$ = effective thread engagement length
• $L_{screw}$ = nominal screw length
• $L_{taper}$ = point taper length (Y from dimension tables)

Minimum effective engagement is 3–4 full thread pitches in plastic substrates. Below this, pull-out strength drops sharply.

Boss Wall Thickness (minimum recommended for plastic):

$$t_{wall} \geq 0.8 \times D_{major}$$

Where:

• $t_{wall}$ = boss wall thickness
• $D_{major}$ = screw major diameter

For brittle plastics (acrylic, phenol formaldehyde), increase this to:

$$t_{wall} \geq 1.0 \times D_{major}$$

Pilot Hole Diameter Selection Principle:

The pilot hole diameter for BF/BT in plastic is not the minor diameter — it is larger, because the cutting action needs room to form the chip:

$$D_{pilot} \approx D_{minor} + (0.10 \text{ to } 0.15 \times P)$$

Where:

• $D_{pilot}$ = pilot hole diameter
• $D_{minor}$ = screw minor diameter
• $P$ = thread pitch

This relationship explains why hole size data tables scale with both screw size and material thickness — thicker sections require slightly larger holes to reduce driving torque and internal stress.

Common Failure Modes and Prevention

Final Checklist Before You Place an Order

Before specifying or ordering BF or BT screws, confirm:

• Substrate is brittle or rigid (not resilient) — if resilient, Type B may be more appropriate
• BF vs. BT decision made based on plastic hardness and torque requirements
• Nominal size and pitch selected from ANSI/ASME B18.6.5M-1986 availability table
• Nominal length confirmed against minimum practical length for chosen head style
• Pilot hole size specified from correct material column and material thickness row
• Clearance hole size specified (if applicable) using appropriate clearance grade
• Material and finish specified in full designation per standard format
• Head style and drive type specified (pan, hex, flat csunk, etc.; slotted vs. cross-recessed)
• Assembly trial completed with representative production material before full-volume order
• Drive torque validated against torsional strength data

What To Do Next

Every plastic assembly you work on from this point forward should have a documented fastener type justification. Not just the size — the type.

If you are reviewing existing designs that use Type B screws in plastic housings and those designs have a field failure history of cracked bosses or stripped threads, the screw type is likely the culprit.

Pull the part. Measure the boss. Check the hole size. Run a comparison with BF or BT. The material cost difference is negligible. The field failure cost is not.

The question to ask yourself right now: In your current project or product, did you verify that your plastic self-threading screw type matches the ductility of your specific substrate — or did you select it based on size alone?

That single question is worth more than any catalogue search.

Standards referenced: ANSI/ASME B18.6.5M-1986 (Metric Thread Forming and Thread Cutting Tapping Screws); ANSI B18.6.4-1981 (R1991) (Self-Tapping and Metallic Drive Screws). All dimensional data preserved from source standard. Dimensions in metric tables are in millimeters unless stated. Dimensions in inch tables are in decimal inches unless stated.