Self-Threading Screws, Thread Inserts and Hole Design: Every Type, Every Thread, Every Application

Every Type, Every Thread, Every Application

A story about a costly mistake — and the complete technical reference that would have prevented it.

Table of Contents

1. What Self-Threading Screws Actually Are
2. The Two Fundamental Categories: Forming vs. Cutting
3. Complete Type-by-Type Reference
4. Head Types and Their Applications
5. Method of Designation — The Correct Way to Order
6. Cross Recesses: Types I, IA, II, and III
7. Thread and Point Dimensions: Types AB, A, and U
8. Thread and Point Dimensions: Types B and BP
9. Thread and Point Dimensions: Types D, F, G, and T
10. Hole Size Reference Tables
11. Types BF and BT Thread Cutting Screws
12. Type U Hardened Metallic Drive Screws
13. Torsional Strength Requirements
14. Self-Tapping Thread Inserts
15. Metric Self-Threading Screws (ANSI/ASME B18.6.5M)
16. Master Decision Matrix: Choosing the Right Screw

What Self-Threading Screws Actually Are

A self-threading screw — also called a self-tapping screw — does something no standard machine screw can do: it creates its own mating thread as it is driven into an unthreaded hole.

This single capability transforms the economics of assembly. There is no pre-tapping step. There is no separate tap tooling. In thin sheet metal, plastics, castings, and composite panels, self-threading screws reduce labor, reduce part count, and when selected correctly, deliver pull-out resistance that rivals tapped thread assemblies.

The governing American standard is ANSI B18.6.4-1981 (R1991) for inch-series screws, and ANSI/ASME B18.6.5M-1986 for metric series.

The Mechanics Behind Thread Creation

Two distinct mechanisms create the thread:

Displacement (Thread Forming): The screw displaces material radially outward and inward as it advances. No material is removed. The result is a work-hardened thread in the parent material with high fatigue resistance and excellent resistance to loosening under vibration. The compressive stress induced around the thread actually strengthens the joint.

Cutting (Thread Cutting): The screw removes material as it advances, depositing chips into flutes or slots on the entering threads. Lower driving torque than forming screws. Preferred when the base material is brittle or when disruptive internal stresses are undesirable.

The Two Fundamental Categories: Forming vs. Cutting

Every self-threading screw falls into one of these two mechanical families. This choice must be made before any other specification.

Thread Forming Screws (Displacing Action)

Critical Note on Type A: The standard explicitly states Type A screws are no longer recommended. In new designs, use Type AB. In existing designs, substitute Type AB wherever possible. the practitioner's mistake was using Type A screws where the engineering drawings specified Type AB — a seemingly small difference that cost him dearly.

Thread Cutting Screws (Cutting Action)

Type U: The Special Case

Type U is the metallic drive screw — a unique category that belongs to neither forming nor cutting in the conventional sense. It features:

• Multiple threads with large helix angle
• Pilot point
• Driven by pressure (not rotational torque)
• Intended for permanent fastenings only (non-removable by design)
• Used in metal and plastics

The ANSI standard governs Type U specifically because it is not a conventional fastener — it is a driven fastener that swages material as it is forced in.

Complete Type-by-Type Reference

The Hero's Journey of Each Screw Type

Understanding why each type exists reveals a progressive evolution of engineering thinking. The progression from Type A to Type AB is the fastener industry's answer to a chronic real-world failure mode: gimlet-point screws driving too aggressively through thin sheet metal and cracking it at the point.

Here is the full engineering logic behind each type:

Type AB — The Universal Workhorse

Geometry: Spaced-thread, gimlet point. Same pitch as Type B.

Why it replaced Type A: Type AB has the same thread spacing as Type B but retains the self-starting gimlet point of Type A. This combination gives it broader material compatibility without the pitch-mismatch risk that made Type A problematic in thin materials.

Best for: Light-gauge sheet metal (steel, stainless, aluminum, brass), resin-impregnated plywood, and thin composite boards.

Type B — The Precision-Fit Performer

Geometry: Spaced-thread, blunt point with tapered entering threads and unfinished crests.

Why it exists: In assemblies where a pilot hole must align the screw precisely, the blunt point (versus the self-starting gimlet) prevents the screw from walking or misaligning as it initiates thread engagement. The slightly finer pitch than Type A improves holding strength in thin-walled materials.

Best for: Thin metal panels, non-ferrous castings (aluminum, zinc), plastics, resin-impregnated plywood, asbestos compositions.

Type BP — The Alignment Specialist

Geometry: Same as Type B with conical point extending beyond incomplete entering threads.

Critical application: When holes are misaligned between mating parts, the extended conical point acts as a dowel to draw components into alignment before threads engage. Also used for piercing fabrics in automotive, marine, and upholstery applications.

Types D, F, G, and T — The Metal Machinists

These four types share thread geometry that approximates machine screw threads, with blunt points and tapered entering threads. Their distinguishing feature is chip cavities and cutting edges that cut rather than displace thread material.

Why four types? The differences lie in the number, geometry, and placement of cutting edges and chip cavities. In practice:

• Type D ("Type 1") — the baseline thread cutting design
• Type F — the only type where tapered threads can be complete at the producer's option
• Type G — variant chip cavity geometry
• Type T ("Type 23") — different cutting edge geometry

For the end user, material compatibility is largely identical across D, F, G, and T. Selection is typically driven by:

1. Chip volume: In deep holes, Type F's optional complete thread reduces chip packing
2. Material hardness: Driving torque varies slightly between types at identical hole sizes
3. Availability: Not all types are stocked in all sizes by all distributors

Head Types and Their Applications

The ANSI standard codifies the following head geometries for self-tapping and metallic drive screws. Head selection affects both mechanical performance and installation tooling.

Standard Head Types

Undercut Heads — A Critical Detail for Short Screws

For short-length flat and oval countersunk head screws, the standard specifies heads undercut to 70 percent of normal side height. This undercut exists to provide adequate thread length on short screws — if the full head height were maintained on a short screw, the thread engagement length would be insufficient for reliable holding strength.

Design Rule: Whenever specifying flat or oval countersunk screws in lengths near the minimum practical length, confirm the undercut condition is acceptable for your seating surface requirements.

Trim Heads — Aesthetic and Functional Downsizing

Flat and oval countersunk trim heads are one or two sizes smaller than the standard head for the same screw size:

• Large Trim Head: One size smaller than standard
• Small Trim Head: Two sizes smaller than standard
• Important: Trim heads are furnished only in cross-recessed types, not slotted

Trim heads are selected when: (a) the counter-bore cannot accept a standard head diameter, or (b) aesthetic minimalism is required in visible assemblies.

Method of Designation — The Correct Way to Order

Ordering the wrong screw type happens when the specification is incomplete. The ANSI standard defines a precise sequence for designation. the practitioner's error — grabbing from the wrong bin — would have been impossible if every screw in the shop were properly labeled according to this system.

Inch-Series Tapping Screws

Sequence: Nominal size → Threads per inch → Nominal length → Point type → Product name (head type + driving provision) → Material → Protective finish (if required)

Examples:

1/4–14 × 1-1/2  Type AB  Slotted Pan Head Tapping Screw, Steel, Nickel Plated
6–32 × 3/4  Type T, Type 1A Cross Recessed Pan Head Tapping Screw, Corrosion Resistant Steel
0.375–16 × 1.50  Type D, Washer Head Tapping Screw, Steel

Metallic Drive Screws (Type U)

Sequence: Nominal size → Nominal length → Product name (head type) → Material → Protective finish (if required)

Note: No threads-per-inch designation because Type U has no conventional thread count — it uses a large-helix-angle multi-start thread.

Examples:

10 × 5/16  Round Head Metallic Drive Screw, Steel
0.312 × 0.50  Round Head Metallic Drive Screw, Steel, Zinc Plated

Metric Tapping Screws

Sequence: Nominal size × Thread pitch × Nominal length → Thread and point type → Product name (head style + driving provision) → Material → Protective finish

Examples:

6.3 × 1.8 × 30  Type AB  Slotted Pan Head Tapping Screw, Steel, Zinc Plated
6 × 1 × 20  Type T, Type 1A Cross Recessed Pan Head Tapping Screw, Corrosion Resistant Steel
4.2 × 1.4 × 13  Type BF, Type 1 Cross Recessed Oval Countersunk Head Tapping Screw, Steel, Chromium Plated
10 × 1.5 × 40  Type D, Hex Flange Head Tapping Screw, Steel

Cross Recesses: Types I, IA, II, and III

The driving recess geometry affects torque transmission, bit engagement, and cam-out resistance. Four types are standardized.

Recess Geometry Descriptions

Practical Selection Guide

Type I and IA are the most common in production environments. The large center opening accommodates minor bit misalignment — critical in high-speed automated assembly where the driver cannot always center perfectly.

Type II (the conventional Phillips configuration) is the legacy standard. The intersecting slot geometry promotes cam-out at a defined torque level, which in the pre-powered-driver era was a feature (preventing over-driving). In modern power tool environments with torque control, cam-out is a defect.

Type III provides the most positive bit engagement of the four types. The square center opening with conical bottom effectively locks the driver bit in place during high-torque applications.

For new designs using power drivers: Specify Type I or Type IA for best cam-out resistance and highest-speed assembly compatibility.

Thread and Point Dimensions: Types AB, A, and U

All dimensions in inches per ANSI B18.6.4-1981 (R1991), Table 4.

Type AB — Thread Forming, Gimlet Point

Note on flat width at crest: Maximum flat width at thread crest shall not exceed 0.004 inch for sizes up to No. 8 inclusive, and 0.006 inch for larger sizes.

Type A — Thread Forming, Gimlet Point (DEPRECATED)

⚠️ Warning: Type A dimensions are shown for reference and legacy replacement purposes only. Do not specify Type A for new designs. Substitute Type AB.

Key difference between A and AB at Size 6: Type A has 18 threads/inch; Type AB has 20 threads/inch. This finer pitch on AB provides improved holding power in thin materials — which is exactly why AB replaced A as the preferred standard.

Type U — Metallic Drive Screw Dimensions

Engineering note on Type U starts: The number of starts (thread leads) increases with screw diameter because larger diameter screws must achieve adequate helix angle for displacement action while maintaining the large helix characteristic that enables pressure-driven installation. A size 3/8 Type U with 12 starts has a dramatically steeper helix angle than a size 00 with 6 starts.

Thread and Point Dimensions: Types B and BP

Per ANSI B18.6.4-1981 (R1991), Table 5. All dimensions in inches.

Types B and BP — Thread Forming

The B thread is the foundation of the modern self-tapping screw family. Key specification notes:

• Flat width at crest: Max 0.004 inch for sizes ≤ No. 8; max 0.006 inch for larger sizes
• Point diameters apply to screw threads before roll threading
• Points are tapered and fluted or slotted
• The BP type shares all B thread dimensions but has an extended conical point

BT flute geometry detail: The flute on Type BT screws has an included angle of 90 to 95 degrees, with the thread cutting edge located above the axis of the screw. Flutes and slots extend through the first full-form thread beyond the taper, except for Type BF screws, on which tapered threads may be complete at the manufacturer's option and flutes may be one pitch short of the first full-form thread.

Thread and Point Dimensions: Types D, F, G, and T

Per ANSI B18.6.4-1981 (R1991), Table 7. All dimensions in inches.

These thread-cutting types use machine screw diameter-pitch combinations. Their thread dimensions differ from Types B and BP because they are designed to approximate standard machine screw thread forms — making them interchangeable with machine screw threads after installation.

Key points for short vs. long screws:

• Screws of nominal lengths equal to or shorter than the listed lengths shall use the short screw point taper length
• Longer screws shall use the long screw point taper length

D, F, G, and T Thread Dimensions (Inch Series)

Hole Size Reference Tables

This section is where most fastener failures originate. The wrong hole size is the single most common cause of:

• Stripped threads during installation
• Inadequate pull-out strength in service
• Thread form distortion in brittle materials
• Excessive driving torque causing screw failure

The hole size tables provided in ANSI B18.6.4 are designed to provide general guidance. Material thickness affects optimal hole size — thinner material needs a slightly smaller hole to ensure adequate thread engagement; thicker material can accommodate a slightly larger hole.

Hole Sizes for Type U Metallic Drive Screws

(In Ferrous and Non-Ferrous Castings, Sheet Metals, Plastics, Plywood, and Fiber)

Hole Sizes for Types A, B/BP/AB, and D/F/G/T in the supplied reference

The torsional strength tables cross-reference to hole size — the correct hole for a given screw type and material will require torque within the allowable driving torque range for that screw's torsional strength rating.

Types A, B, and BP — Drilled or Clean-Punched Holes in Steel Sheet Metal

The following tables cover sizes 2 through 1/4" in steel, stainless steel, Monel, and brass:

Steel, Stainless Steel, Monel, and Brass — Type AB Forming Screws

Size 2 (0.086 in. basic diameter)

Size 4 (0.112 in. basic diameter)

Size 6 (0.138 in. basic diameter)

Size 10 (0.190 in. basic diameter)

Size 1/4 (0.250 in. basic diameter)

Types D, F, G, and T — Thread Cutting Screws in Sheet Metals

All dimensions in inches. These hole sizes cover both steel and aluminum alloy sheets.

Selected Sizes — Steel and Aluminum Alloy

Why aluminum requires a slightly different hole size: Aluminum is softer and more ductile than steel. A slightly smaller hole in aluminum generates sufficient thread engagement with less stress on the parent material — using the steel-sized hole in aluminum would reduce pull-out strength by failing to generate adequate compressive stress in the formed thread.

Types BF and BT Thread Cutting Screws

Application Context

Types BF and BT fill a specific gap: brittle or semi-brittle materials where thread forming is impossible but machine screw pitch spacing is not required. They carry the spaced thread geometry of Type B but add cutting capability — solving the problem of applying self-tapping screws to phenolic resins, filled plastics, and asbestos compositions that would crack under the compressive stress of forming screws.