Self-Threading Screws, Thread Inserts and Hole Design: BF and BT Hole Sizes

BF and BT Hole Sizes — Die Cast Zinc and Aluminum

All dimensions in inches.

BF and BT Hole Sizes — Plastics

The plastic hole size data is organized by plastic type due to vastly different material properties:

The minimum/maximum depth of penetration specification in plastics exists because too-shallow engagement strips threads immediately; too-deep penetration generates excessive hoop stress that cracks brittle plastics. This is not specified for metals because ductile metals accommodate a wider range of engagement without catastrophic failure.

Type U Hardened Metallic Drive Screws

What Makes Type U Fundamentally Different

the practitioner nearly made a second mistake on the same job — he almost specified Type U drive screws for a gasketed cover panel that would need periodic maintenance access. Type U screws, once driven, are not intended for removal. Their large helix angle creates a thread that cold-welds to the parent material under the insertion pressure. Attempting removal strips the parent thread immediately.

The application profile for Type U:

• Nameplates and identification labels driven permanently into housings
• Electrical conduit fittings where vibration resistance and permanence are paramount
• Instrument panel hardware requiring vibration-proof installation
• Any application where disassembly access is not needed

Torsional Strength Requirements

Why Torsional Strength Is Non-Negotiable

A screw that strips its own drive recess or twists off its shank before completing installation is worse than no screw at all. The torsional strength requirement in ANSI B18.6.4 represents the minimum torque the screw shank and drive feature must withstand without failure.

The standard specifies torsional strength requirements based on screw type and size. These minimums are established through proof testing, and they define the lower bound of screw performance that any compliant product must meet.

ANSI Standard Torsional Strength Requirements

(ANSI B18.6.4-1981, R1991) — Values in pound-inches

Critical design insight: At size 10, the torsional strength for Types D/F/G/T fine thread (74 lb-in) is 54% greater than for Type A (48 lb-in). This is not an accident — thread-cutting screws in fine thread configurations generate higher installation torques, so the screw body must be stronger to survive its own installation.

Converting Torsional Strength to Practical Torque Limits

$$T_{max} = T_{spec} \times SF$$

Where:

• $T_{max}$ = maximum allowable tool torque setting
• $T_{spec}$ = ANSI torsional strength from table above (pound-inches)
• $SF$ = safety factor (typically 0.75 for power tool installation; 0.85 for hand tool)

Example: Installing a No. 10 Type AB screw with a power driver:

$$T_{max} = 56 \times 0.75 = 42 \text{ lb-in}$$

Set your driver clutch to 42 lb-in maximum. Anything above this risks shank failure on a borderline-compliant screw.

Self-Tapping Thread Inserts

When to Use an Insert Instead of a Screw

The self-tapping thread insert is an engineering bridge between self-tapping screws and conventional machine screws. It is, in essence, a hard bushing with internal standard threads and external self-tapping capability.

Internal thread conformance: Unified and American Standard classes 2B and 3B, depending on insert type.

External thread: Has cutting edges on the entering end — this is the self-tapping mechanism.

Materials available:

• Case-hardened carbon steel
• Stainless steel
• Brass (designed specifically for wood installation)

Base material compatibility: Magnesium, aluminum, cast iron, zinc, plastics, wood, and other soft materials.

Why Inserts Exist — The Strength Gap Problem

In aluminum or die-cast zinc, a direct-tapped machine screw thread achieves only a fraction of the thread stripping strength you can get in steel. A self-tapping thread insert solves this by:

1. Cutting strong threads in the soft base material using a large-diameter external thread
2. Providing an internal thread that accepts standard machine screws or studs at full strength class

The result: machine-screw-strength connections in materials that could never support machine screw loads by direct tapping alone.

Screw Thread Inserts (Helical Coil Type)

The Heli-Coil type is the archetypal helical thread insert — a coil of diamond-shaped wire (stainless steel or phosphor bronze) that:

• Screws into a specially tapped hole
• Forms a continuous mating internal thread
• Provides a much stronger thread than the base material alone

Available range:

• Coarse thread: 4–40 through 1-1/2–6 (Unified/National series)
• Fine thread: 6–40 through 1-1/2–12

Thread class compliance: With appropriate taps and gages, helical inserts Consider an engineering practitioner 2, 2B, 3, and 3B requirements.

Boss diameter design: Conventional boss diameter and edge distance design practice applies. The major diameter of a hole tapped to receive a helical insert is not substantially larger than the major diameter of the thread the insert provides — so standard boss sizing does not need modification.

Metric Self-Threading Screws

Governing Standard: ANSI/ASME B18.6.5M-1986

The metric tapping screw standard mirrors the philosophy of B18.6.4 but uses SI units throughout and provides metric-specific thread pitch combinations from the coarse thread series.

Metric Thread Types — Summary

Thread Forming Types

Type AB: Spaced thread, gimlet point — thin metal, resin-impregnated plywood, asbestos compositions.

Type B: Spaced thread, blunt point with tapered entering threads and unfinished crests — thin metal, non-ferrous castings, resilient plastics, resin-impregnated plywood, asbestos.

Thread forming types are selected when large internal stresses are permissible or desirable (they increase resistance to loosening under vibration).

Thread Cutting Types

Types BF and BT: Spaced threads, blunt point, tapered entering threads, one or more cutting edges or chip cavities — plastics, asbestos, and similar materials.

Types D, F, and T: Machine screw diameter-pitch combinations (metric coarse series) approximating 60° basic thread form, blunt point, tapered entering threads, cutting edges and chip cavities — aluminum, zinc, lead die castings; steel sheets and shapes; cast iron; brass; plastics.

Thread cutting types are selected when disruptive internal stresses are undesirable, or when excessive driving torques are encountered with forming types.

Metric Type AB and B — Thread and Point Dimensions

Per ANSI/ASME B18.6.5M-1986, Table 3. All dimensions in millimeters.

Minimum effective grip length for Type AB: Subtract the Point Length Factor from the minimum screw length. A 2.2 × 0.8 screw at its minimum length of 4 mm with a 1.6 mm point length factor has a minimum effective grip of 2.4 mm — barely adequate for materials thinner than 0.5 mm. Size up to the next screw length for thin materials near minimum.

Metric Types BF, BT, D, F, and T — Thread Dimensions

Per ANSI/ASME B18.6.5M-1986, Table 4. All dimensions in millimeters.

Types BF and BT

Types D, F, and T (Metric Thread Cutting)

Metric Hole Sizes — Type AB in Steel Sheet Metal (Drilled or Clean-Punched)

All dimensions in millimeters. Sample data covering the most common screw sizes.

In Steel, Stainless Steel, Monel, and Brass

In Aluminum Alloy

Metric Clearance Holes

ANSI/ASME B18.6.5M-1986 provides clearance hole diameters for three fit categories. Normal clearance is preferred for most applications.

When to use close clearance vs. loose clearance:

• Close: Critical alignment of assembled components, restricted wall thickness. Note: countersinking or counterboring at the fastener entry may be needed for proper head seating.
• Normal (Preferred): Standard applications. Balances ease of assembly with positional accuracy.
• Loose: Maximum adjustment capability is needed — sliding or slotted holes where one component must shift relative to the other before final tightening.

Material and Heat Treatment

Metric tapping screws per B18.6.5M are normally fabricated from carbon steel and processed to meet performance requirements. Additional available materials:

• Corrosion resistant steel
• Monel
• Brass
• Aluminum alloys

Material properties and performance characteristics for non-carbon-steel screws must be mutually agreed upon between manufacturer and purchaser, as the standard's performance requirements are written for carbon steel.

Master Decision Matrix: Choosing the Right Screw

The following matrix consolidates all the preceding information into a single selection framework. the practitioner needed this on that Tuesday afternoon.

Step 1 — Determine the Base Material

Step 2 — Select the Specific Type

Step 3 — Confirm Thread Parameters

$$\text{Engagement Length} \geq 2 \times d$$

Where $d$ = nominal screw diameter (inch or mm).

Two diameters of thread engagement is the minimum for reliable pull-out strength in ductile materials. For brittle materials (plastics, cast iron), increase to $3 \times d$ minimum.

Step 4 — Select Hole Size

Use the appropriate hole size table from this guide based on:

1. Screw type (forming vs. cutting)
2. Base material (steel, aluminum, plastic, cast metal)
3. Material thickness

Step 5 — Verify Torsional Margin

$$T_{driver} \leq 0.75 \times T_{spec}$$

Where $T_{spec}$ is from the ANSI torsional strength table. Never set a power driver above 75% of the rated torsional strength — screws at the lower manufacturing tolerance limit will fail at the rated value, not above it.

Step 6 — Complete the Designation

Use the ANSI designation format precisely. Incomplete designations cause procurement errors — the most expensive mistakes in fastener engineering are not made at installation, they are made when ordering.

the practitioner's Epilogue

the practitioner eventually won back the client — not with cheaper fasteners or faster timelines, but with a complete fastener specification package that documented every hole size, every torque setting, and every screw designation for the full product range. When the client's engineer reviewed it, he called it the most thorough fastener specification he had received from any vendor.

The technical knowledge the practitioner assembled after his failure is exactly what this guide contains. The screw types he confused — A versus AB — differ by just a few threads per inch. But in engineering, "close enough" is a failure mode.

Every number in this guide has a consequence attached to it. The correct hole diameter is not an academic detail — it is the difference between a joint that holds for twenty years and one that strips on first installation.

What's Your Screw Selection Challenge?

Three questions that will sharpen your specification process:

1. Are you still using Type A screws? Review your inventory and engineering drawings. Any Type A designation should be replaced with Type AB per the current ANSI standard recommendation.

2. Do your hole size tables account for material thickness? Many engineers use a single hole size for a given screw regardless of thickness. The ANSI tables show hole sizes varying by as much as 15% across the thickness range for a single screw size — that variability matters.

3. Have you set torsional limits on your power drivers? Most shops run power drivers at a fixed torque setting that was "set once and never changed." Use the torsional strength table in this guide to recalculate your limits for each screw type you run.

The engineer who questions their defaults is the engineer who never loses a contract to a stripped hole.

Standards referenced: ANSI B18.6.4-1981 (R1991); ANSI/ASME B18.6.5M-1986. Always verify against the current published edition of the governing standard for production engineering decisions.

The Hidden Language of Self-Threading Screws: A Complete Engineer's Guide to Thread Inserts, Tapping Screws, Hole Sizing, and Material Mastery

A story of stripped threads, failed assemblies, and the mastery that comes from finally understanding the fastener nobody talks about.

What You'll Master in This Guide

By the time you finish reading, you'll be able to:

• Identify every ANSI/metric self-threading screw type and select the right one for your material
• Choose the correct pilot hole size for every material, thickness, and screw combination — with actual specification tables
• Understand thread inserts (both self-tapping and Heli-Coil types) and know exactly when to use them
• Specify nominal screw lengths correctly for both 90° and countersunk heads
• Apply material and heat treatment knowledge to avoid failure under torque
• Use clearance hole and clean-punched hole data with confidence, in both inch and metric systems

The Foundation — What Is a Self-Threading Screw, Really?

Before the practitioner could fix his problem, he had to understand what a self-threading screw actually does at the material level.

A self-threading screw is not simply "a screw that makes its own hole." That's a dangerous oversimplification. It is a precision fastener engineered to create or cut its own mating thread within a pre-drilled pilot hole of a specific size. The two words — "forming" and "cutting" — define entirely different mechanical behaviors.

┌─────────────────────────────────────────────────────────────────┐
│              SELF-THREADING SCREW CLASSIFICATION                │
│                                                                 │
│  THREAD FORMING                    THREAD CUTTING               │
│  (Displacing Action)               (Cutting Action)             │
│                                                                 │
│  ▸ Types AB, B, BP, C, U           ▸ Types D, F, G, T           │
│                                    ▸ Types BF, BT               │
│                                                                 │
│  Material displaced into           Material removed as          │
│  thread form = stronger            chip = less stress           │
│  grip, no chips                    in brittle materials         │
│                                                                 │
│  Best for: ductile metals,         Best for: hard metals,       │
│  plastics, thin sheet              cast metals, plastics        │
└─────────────────────────────────────────────────────────────────┘

This distinction alone resolves the practitioner's mystery. He had switched to a harder aluminum alloy. Thread-forming screws in hard alloy create excessive internal stress. The correct choice was a thread-cutting type — specifically Type D, F, or T — which removes material rather than compressing it.

Every ANSI Type Explained — The Complete Reference

The governing standard for inch-series self-threading screws is ANSI B18.6.4-1981 (R1991). For metric, it is ANSI/ASME B18.6.5M-1986. These standards define eleven distinct screw types. You need to know all of them.

Thread-Forming Types (Displacing Action)

Type A A spaced-thread screw with a gimlet point, originally designed for light sheet metal, resin-impregnated plywood, and asbestos compositions. This type is no longer recommended. Substitute Type AB in all new designs and wherever possible in existing designs.

Type AB (Formerly BA) The direct replacement for Type A. Same pitch as Type B but with a gimlet point. The current standard for thin metal, resin-impregnated plywood, and asbestos compositions. When in doubt between A and AB, always choose AB.

Type B A spaced-thread screw with a blunt point and generally finer pitches than Type A. Used for thin metal, non-ferrous castings, plastics, resin-impregnated plywood, and asbestos compositions. The blunt point makes it safer in automated assembly.

Type BP Identical to Type B but with a conical point that extends beyond the incomplete entering threads. The extended point excels at piercing fabrics and at threading into assemblies where holes may be slightly misaligned.

Type C Screws with machine screw diameter-pitch combinations, approximately Unified thread form, with blunt tapered points. Used where a machine screw thread profile is preferred over spaced-thread types. However, Type C requires high driving torques and is not recommended for new designs due to declining use compared to more efficient thread-tapping alternatives.

Type U — Metallic Drive Screws A multiple-threaded drive screw with a large helix angle and pilot point. Driven by pressure, not rotation. Type U is forced into the work and is designed exclusively for permanent fastenings. Do not use it in applications requiring removal. It creates multiple threads simultaneously through a displacing action.

Thread-Cutting Types (Cutting Action)

Types D, F, G, and T These are the workhorses of the thread-cutting family. They feature machine screw diameter-pitch combinations approximating machine screw threads, blunt points, tapered entering threads, and one or more cutting edges with chip cavities. Types D, F, G, and T differ in their chip cavity geometry and taper detail (Type F's tapered threads may be complete or incomplete at the producer's option; all others have incomplete tapered threads).

Ideal materials for D, F, G, T:

• Aluminum, zinc, and lead die-castings
• Steel sheets and shapes
• Cast iron
• Brass
• Plastics

Types BF and BT Thread-cutting screws with spaced threads (as in Type B) but fitted with blunt points and one or more cutting grooves. Designed specifically for:

• Plastics
• Asbestos compositions
• Similar soft or granular materials

where the spaced thread provides better holding power than a machine-thread pitch, but where the cutting action prevents the cracking and distortion that thread-forming types would cause.

Quick-Reference Type Selection Matrix

Head Types — The Interface Between Screw and World

The head of a tapping screw is not decorative. It defines the bearing surface, the wrenching capability, and the flush-mount potential. ANSI B18.6.4-1981 recognizes these primary head types:

Round Head Semi-elliptical top surface, flat bearing surface. A classic general-purpose head.

Pan Head The current preferred alternative to the round head. Lower profile with a larger bearing surface.

Fillister Head Rounded top, cylindrical sides, flat bearing. Good for applications where the head must be recessed.

Hex Head Flat or indented top, six flat sides, flat bearing. The hex wrenching capability far exceeds any slotted version. Note: slotted hex heads are not recommended for new designs — the secondary operation to cut the slot often produces burrs that interfere with socket wrench engagement, defeating the very advantage of the hex form.

Hex Washer Head Six flat sides formed integrally with a projecting flat washer, providing a wide bearing surface. The same caveat as slotted hex applies: slotted hex washer heads are not recommended for new designs.

Truss Head Low rounded top with a flat bearing surface whose diameter is larger than a standard round head. Despite its wide bearing area, not recommended for new designs — it is an inherently weak design.