The Finish Factor — How Plating Affects Performance
Finish matters more than most engineers realize until a production run fails.
The hole size tables in ANSI/ASME B18.6.5M were developed using plain unfinished carbon steel screws. Experience has shown that these holes are suitable for screws with most types of commercial finishes.
However, be aware of the following:
- Lubricity differences between finishes mean installation torques may need adjustment even when the same hole size is used. A zinc-plated screw will drive differently than a phosphate-and-oil-coated screw.
- Exceptionally heavy finishes (thick plating, powder coating, galvanizing) may require slightly different hole sizes to compensate for the added diameter.
- Screws assembled into materials of higher hardness than the test standard may require hole size adjustment.
The definitive test: When in doubt, test three sample assemblies in the actual production material with the actual finish. The spec tables are starting points, not final answers for demanding applications.
Master Reference Card — The Complete Quick-Reference System
ANSI Standard Cross-Reference
| Standard | Scope |
|---|---|
| ANSI B18.6.4-1981 (R1991) | Sheet metal, self-tapping, and metallic drive screws (inch) |
| ANSI/ASME B18.6.5M-1986 | Metric thread forming and thread cutting tapping screws |
Screw Type — At a Glance
| Type | Action | Preferred for | Chips? | Removable? | Status |
|---|---|---|---|---|---|
| A | Forming | Light sheet, plywood | No | Yes | Discontinued — use AB |
| AB | Forming | Light sheet, plywood, asbestos | No | Yes | Current |
| B | Forming | Thin metal, non-ferrous cast, plastics | No | Yes | Current |
| BP | Forming | Fabrics, misaligned holes | No | Yes | Current |
| C | Forming | Machine thread in thin material | No | Yes | Not recommended |
| D, F, G, T | Cutting | Harder metals, cast, die cast, brass | Yes | Yes | Current |
| BF, BT | Cutting | Plastics, asbestos | Yes | Yes | Current |
| U | Displacing | Permanent fastenings only | No | No | Current |
Key Formulas
Torsional Strength Required (minimum):
Effective Thread Engagement — Pan/Hex Head:
Effective Thread Engagement — Countersunk Head:
Tap Drill Size for Thread Inserts (Unified form):
Where is the basic major diameter, is threads per inch, and is the desired percentage of full thread depth expressed as a decimal.
The Universal Takeaway — For Every Engineer, Machinist, and Designer
the practitioner's problem was never really about the screws. It was about the assumption that general knowledge is specific enough.
Every time a self-threading screw fails — strips a housing, backs out under vibration, shears during installation — it fails for a reason that was entirely predictable. The hole was the wrong size. The type was wrong for the material. The screw was too soft, or too short, or specified incorrectly on the drawing.
The information to prevent every one of these failures exists. It existed before the practitioner's first prototype. It existed before yours.
The difference between a professional and an expert isn't the quality of the tools they use. It's the depth at which they understand why those tools work — and at what precise point they stop working.
Self-threading screws are deceptively simple objects. A pointed piece of hardened metal that bites into softer material and holds fast. But behind that simplicity is an entire engineering language: types, geometries, torsional limits, material pairings, hole dimensions calibrated to the thousandth of an inch.
Fluency in that language is what separates the engineer who sends six units back to the customer from the one who never receives the call.
Your Next Step
Take one fastener from your current design or production floor — just one — and trace it through every decision in this guide:
- Is the type correct for your material's ductility and hardness?
- Is your hole size from the correct table for your specific material, thickness, and hole-preparation method?
- Is your nominal length sufficient for your head style and required thread engagement?
- Is your screw material harder than the material it's threading into?
If you can answer all four questions with citations from specification tables — not from habit, not from approximation, but from verified engineering data — then you have achieved what the practitioner achieved after six returned units and a very expensive lesson.
If you can't answer all four yet, you now have the reference to find every answer. Start with the hole size. That's where most failures begin.
Standards referenced: ANSI B18.6.4-1981 (R1991), ANSI/ASME B18.6.5M-1986. Dimensional data extracted from engineering reference handbooks. All specifications should be verified against the current revision of the applicable standard before production use.
The Plastic Prototype Disaster: What Every Engineer Must Know About Metric BF and BT Self-Threading Screws
"The right screw in the wrong hole is just a slower way to fail."
The Scene: A Product Launch on the Brink
The prototype was beautiful.
Twelve months of development. A sleek consumer electronics housing made from polycarbonate and ABS plastic. Snap-fit joints, flush surfaces, and an assembly designed to be produced in volume.
the practitioner, a junior mechanical engineer at a mid-sized contract manufacturer, had specified the fasteners from a standard catalogue. He chose a metric thread-forming screw — Type B — because it was listed for plastics and it was the same size as something he had used on an aluminum bracket the week before.
On the first assembly run, the production team reported that every third screw was either cracking the boss wall or spinning out without torque.
The product launch was three weeks away.
The problem was not the size of the screw. The problem was the type.
the practitioner had used a thread-forming screw in a material that needed a thread-cutting screw. And he had no idea there was a difference.
Failure trigger and engineering context
Self-threading screws fall into two fundamentally different categories, and confusing them is the single most common and costly mistake in plastics assembly.
| Category | Mechanism | Best For |
|---|---|---|
| Thread-Forming (Types AB, B, BP) | Displaces material to create a thread | Softer metals, resilient plastics, plywood |
| Thread-Cutting (Types BF, BT, D, F, T) | Removes material (cuts chips) to create a thread | Brittle plastics, thermosets, asbestos compositions, die castings |
Thread-forming screws generate internal stress. They push material outward as the thread is formed. In ductile or resilient materials, this stress is actually beneficial — it grips the fastener tightly and resists loosening.
But in brittle or rigid plastics — phenol formaldehyde, polycarbonate, acrylic, styrene resin — that same stress becomes a crack initiation site. The boss wall fractures. The fastener strips out. The assembly fails.
Thread-cutting screws eliminate that stress. Their cutting edges or chip cavities shave material away cleanly, like a tap, leaving a formed thread without internal pressure.
This is exactly where Types BF and BT enter the story.
Understanding the Protagonists: Types BF and BT Defined
Per ANSI/ASME B18.6.5M-1986 (the governing standard for metric thread forming and thread cutting tapping screws), Types BF and BT are defined as:
Spaced threads with blunt point and tapered entering threads having unfinished crests, as on Type B, with one or more cutting edges or chip cavities, intended for use in plastics, asbestos compositions, and other similar materials.
In plain language: they look like a Type B screw on the outside — spaced threads, blunt point — but they have one or more cutting grooves or flutes machined into the tapered entry section that act as cutting edges to remove material rather than displace it.
The Structural Difference: BF vs. BT
Both types share the same fundamental design philosophy (thread cutting, spaced thread, blunt point), but they differ in the geometry of their cutting feature:
| Feature | Type BF | Type BT |
|---|---|---|
| Cutting feature | Slot/flute — tapered threads may be complete at manufacturer's option; flutes may be one pitch short of first full form thread | Flute with included angle of 90–95 degrees; thread cutting edge located above the axis of the screw |
| Flute extent | Through first full form thread beyond taper (flute may be one pitch short) | Through first full form thread beyond taper |
| Thread style | Spaced (same pitch as Type B) | Spaced (same pitch as Type B) |
| Point | Blunt, tapered entering threads with unfinished crests | Blunt, tapered entering threads with unfinished crests |
| Primary application | Plastics, asbestos compositions, similar materials | Plastics, asbestos compositions, similar materials |
The BT flute geometry is more aggressive. With its 90–95° included angle and the cutting edge positioned above the screw's axis, BT produces a more positive cutting action. BF offers a gentler cut, sometimes preferred in softer plastic compositions.
Practical rule of thumb: When in doubt between BF and BT, prototype both. The harder and more brittle the plastic, the more BT's aggressive geometry helps. In softer thermoplastics, BF often produces a cleaner result with less chip volume.
Standard Reference
All metric BF and BT screws are governed by:
ANSI/ASME B18.6.5M-1986 — American National Standard for Metric Thread Forming and Thread Cutting Tapping Screws
The same standard also covers Types AB, B, D, F, and T for completeness.
Thread and Point Dimensions: Complete Technical Reference
Types BF and BT — All Dimensions in Millimeters
The following table presents the complete thread and point dimensional data per ANSI/ASME B18.6.5M-1986. All values apply to screw blanks prior to roll threading.
Notation key:
| Symbol | Meaning |
|---|---|
| D1 | Thread Major Diameter |
| D2 | Thread Minor Diameter |
| D3 | Point Diameter (before roll threading) |
| Y | Point Taper Length (Type B limits; max ≈ 2× thread pitch) |
| L | Minimum Practical Nominal Screw Length |
Body diameter rule (BF and BT): The body diameter (unthreaded portion) shall be not less than the minimum minor diameter, nor greater than the maximum major diameter of the thread.
| Nominal Size × Pitch (mm) | Basic Screw Dia. (Ref) | Basic Thread Pitch (Ref) | D1 Major Dia. Max | D1 Major Dia. Min | D2 Minor Dia. Max | D2 Minor Dia. Min | D3 Point Dia. Max | D3 Point Dia. Min | Y Point Taper Length Max | Y Point Taper Length Min | L Min (Pan/Hex/Hex Flange Head) | L Min (Flat/Oval Csunk Head) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2.2 × 0.8 | 2.184 | 0.79 | 2.24 | 2.10 | 1.63 | 1.52 | 1.47 | 1.37 | 1.6 | 1.2 | 4 | 5 |
| 2.9 × 1 | 2.845 | 1.06 | 2.90 | 2.76 | 2.18 | 2.08 | 2.01 | 1.88 | 2.1 | 1.6 | 5 | 7 |
| 3.5 × 1.3 | 3.505 | 1.27 | 3.53 | 3.35 | 2.64 | 2.51 | 2.41 | 2.26 | 2.5 | 1.9 | 6 | 8 |
| 4.2 × 1.4 | 4.166 | 1.41 | 4.22 | 4.04 | 3.10 | 2.95 | 2.84 | 2.69 | 2.8 | 2.1 | 7 | 10 |
| 4.8 × 1.6 | 4.826 | 1.59 | 4.80 | 4.62 | 3.58 | 3.43 | 3.30 | 3.12 | 3.2 | 2.4 | 8 | 11 |
| 5.5 × 1.8 | 5.486 | 1.81 | 5.46 | 5.28 | 4.17 | 3.99 | 3.86 | 3.68 | 3.6 | 2.7 | 9 | 12 |
| 6.3 × 1.8 | 6.350 | 1.81 | 6.25 | 6.03 | 4.88 | 4.70 | 4.55 | 4.34 | 3.6 | 2.7 | 10 | 13 |
| 8 × 2.1 | 7.938 | 2.12 | 8.00 | 7.78 | 6.20 | 5.99 | 5.84 | 5.64 | 4.2 | 3.2 | 12 | 17 |
| 9.5 × 2.1 | 9.525 | 2.12 | 9.65 | 9.43 | 7.85 | 7.59 | 7.44 | 7.24 | 4.2 | 3.2 | 14 | 19 |
Design Note: For the 9.5 × 2.1 screw, the major diameter maximum (9.65 mm) intentionally exceeds the nominal size — this is correct per the standard and reflects the spaced thread profile geometry.
The Head Types Available for BF and BT Metric Screws
Per ANSI/ASME B18.6.5M-1986, the head types applicable to metric BF and BT tapping screws include:
- Flat Countersunk Head — flat top, conical bearing surface, 90–92° head angle
- Oval Countersunk Head — rounded top, conical bearing surface, 90–92° head angle
- Pan Head — flat or rounded top blending into cylindrical sides, flat bearing surface (slotted or recessed)
- Hex Head — flat or indented top, six flat sides, flat bearing surface
- Hex Flange Head — flat or indented top, six flat sides, integral frustroconical or convex flange providing a flat bearing surface
The minimum practical screw length differs between head families:
- Pan, Hex, and Hex Flange Heads use the shorter minimum lengths shown in the table above
- Flat and Oval Countersunk Heads require slightly longer minimum lengths
Recommended Nominal Screw Lengths — Length-Diameter Selection Guide
Per ANSI/ASME B18.6.5M-1986, not all length-diameter combinations are recommended. Use this availability matrix for BF and BT types:
| Nominal Screw Length (mm) | 2.2 | 2.9 | 3.5 | 4.2 | 4.8 | 5.5 | 6.3 | 8 | 9.5 |
|---|---|---|---|---|---|---|---|---|---|
| 4 | PH | — | — | — | — | — | — | — | — |
| 5 | PH | — | — | — | — | — | — | — | — |
| 6 | A | A | — | — | — | — | — | — | — |
| 8 | A | A | A | — | — | — | — | — | — |
| 10 | A | A | A | A | A | — | — | — | — |
| 13 | A | A | A | A | A | A | A | — | — |
| 16 | A | A | A | A | A | A | A | — | — |
| 20 | A | A | A | A | A | A | — | — | — |
| 25 | A | A | A | A | A | A | A | — | — |
| 30 | A | A | A | A | A | — | — | — | — |
| 35 | A | A | A | A | A | — | — | — | — |
| 40 | — | — | — | A | A | A | A | — | — |
| 45 | — | — | — | — | — | A | A | — | — |
| 50 | — | — | — | — | — | A | A | — | — |
| 55 | — | — | — | — | — | — | A | — | — |
| 60 | — | — | — | — | — | — | A | — | — |
Key: A = Available | PH = Pan/Hex head only | — = Not in standard range
Installation Hole Sizes: The Make-or-Break Variable
Here is where the practitioner's story becomes your education. Hole size is everything.
Too small → cracked boss walls and stripped drive recesses Too large → stripped threads and zero pull-out strength Just right → clean thread cutting, proper engagement, rated performance
Per ANSI/ASME B18.6.5M-1986, hole size data for BF and BT types is provided for the following material categories:
- Die Cast Zinc and Aluminum
- Phenol Formaldehyde (thermoset plastic)
- Cellulose Acetate and Nitrate, Acrylic and Styrene Resins (thermoplastics)
Critical Note on Finish Effects
The finish (plating or coating) on metric tapping screws and the material composition and hardness of the mating component are factors that affect assembly torques. Recommended hole sizes were based on plain unfinished carbon steel metric tapping screws. Screws with commercial finishes generally work with specified holes, but heavy finishes or harder substrate materials may require hole size adjustment. The necessity and extent of such deviations is best determined by experiment in the particular assembly environment.
This means: always validate hole size with assembly trials, especially when switching to plated or coated fasteners.
Hole Sizes: Die Cast Zinc and Aluminum (All dimensions in mm)
| Nominal Size × Pitch | Material Thickness | Hole Size | Drill Size |
|---|---|---|---|
| 2.2 × 0.8 | 1.52 | 1.85 | 49 |
| 2.2 × 0.8 | 2.11 | 1.85 | 49 |
| 2.2 × 0.8 | 2.77 | 1.93 | 48 |
| 2.2 × 0.8 | 3.18 | 1.93 | 48 |
| 2.2 × 0.8 | 3.56 | 1.93 | 48 |
| 2.9 × 1 | 2.77 | 2.49 | 40 |
| 2.9 × 1 | 3.18 | 2.54 | 39 |
| 2.9 × 1 | 3.56 | 2.54 | 39 |
| 2.9 × 1 | 4.78 | 2.54 | 39 |
| 2.9 × 1 | 6.35 | 2.59 | 38 |
| 3.5 × 1.3 | 3.18 | 3.05 | 31 |
| 3.5 × 1.3 | 3.56 | 3.05 | 31 |
| 3.5 × 1.3 | 4.78 | 3.05 | 31 |
| 3.5 × 1.3 | 6.35 | 3.18 | — |
| 3.5 × 1.3 | 7.92 | 3.18 | — |
| 4.2 × 1.4 | 3.18 | 3.78 | 25 |
| 4.2 × 1.4 | 3.56 | 3.78 | 25 |
| 4.2 × 1.4 | 4.78 | 3.78 | 25 |
| 4.2 × 1.4 | 6.35 | 3.86 | 24 |
| 4.2 × 1.4 | 7.92 | 3.86 | 24 |
| 4.8 × 1.6 | 3.18 | 4.22 | 19 |
| 4.8 × 1.6 | 3.56 | 4.22 | 19 |
| 4.8 × 1.6 | 4.78 | 4.22 | 19 |
| 4.8 × 1.6 | 6.35 | 4.32 | 18 |
| 4.8 × 1.6 | 7.92 | 4.37 | — |
| 4.8 × 1.6 | 9.52 | 4.37 | — |
| 5.5 × 1.8 | 3.18 | 4.85 | 11 |
| 5.5 × 1.8 | 3.56 | 4.85 | 11 |
| 5.5 × 1.8 | 4.78 | 4.85 | 11 |
| 5.5 × 1.8 | 6.35 | 4.98 | 9 |
| 5.5 × 1.8 | 7.92 | 4.98 | 9 |
| 5.5 × 1.8 | 9.52 | 4.98 | 9 |
| 6.3 × 1.8 | 3.18 | 5.61 | 2 |
| 6.3 × 1.8 | 3.56 | 5.61 | 2 |
| 6.3 × 1.8 | 4.78 | 5.61 | 2 |
| 6.3 × 1.8 | 6.35 | 5.79 | 1 |
| 6.3 × 1.8 | 7.92 | 5.79 | 1 |
| 6.3 × 1.8 | 9.52 | 5.79 | 1 |
| 8 × 2.1 | 3.18 | 7.14 | K |
| 8 × 2.1 | 3.56 | 7.14 | K |
| 8 × 2.1 | 4.78 | 7.14 | K |
| 8 × 2.1 | 6.35 | 7.14 | K |
| 8 × 2.1 | 7.92 | 7.37 | L |
| 8 × 2.1 | 9.52 | 7.37 | L |
| 9.5 × 2.1 | 3.18 | 8.74 | — |
| 9.5 × 2.1 | 3.56 | 8.74 | — |
| 9.5 × 2.1 | 4.78 | 8.74 | — |
| 9.5 × 2.1 | 6.35 | 8.74 | — |
| 9.5 × 2.1 | 7.92 | 8.84 | S |
| 9.5 × 2.1 | 9.52 | 8.84 | S |
Hole Sizes: Phenol Formaldehyde (Thermoset Plastic)
All dimensions in mm. Depth of penetration values define the engagement range for blind holes.
| Nominal Size × Pitch | Hole Size | Drill Size | Penetration Min (mm) | Penetration Max (mm) |
|---|---|---|---|---|
| 2.2 × 0.8 | 1.98 | — | 2.39 | 6.35 |
| 2.9 × 1 | 2.64 | 37 | 3.18 | 7.92 |
| 3.5 × 1.3 | 3.18 | — | 4.78 | 9.52 |
| 4.2 × 1.4 | 3.73 | 26 | 6.35 | 12.70 |
| 4.8 × 1.6 | 4.32 | 18 | 7.92 | 15.88 |
| 5.5 × 1.8 | 4.93 | 10 | 9.52 | 15.88 |
| 6.3 × 1.8 | 5.79 | 1 | 9.52 | 19.05 |
Why phenol formaldehyde needs its own column: Phenol formaldehyde (Bakelite and its relatives) is the most brittle of common engineering plastics. It is extremely sensitive to hoop stress. Hole sizes here are slightly larger than for thermoplastics to compensate for the material's zero tolerance for internal stress.
Hole Sizes: Cellulose Acetate/Nitrate, Acrylic, and Styrene Resins (Thermoplastics)
All dimensions in mm.
| Nominal Size × Pitch | Hole Size | Drill Size | Penetration Min (mm) | Penetration Max (mm) |
|---|---|---|---|---|
| 2.2 × 0.8 | 1.93 | 48 | 2.39 | 6.35 |
| 2.9 × 1 | 2.54 | 39 | 3.18 | 7.92 |
| 3.5 × 1.3 | 3.05 | 31 | 4.78 | 9.52 |
| 4.2 × 1.4 | 3.66 | 27 | 6.35 | 12.70 |
| 4.8 × 1.6 | 4.22 | 19 | 7.92 | 15.88 |
| 5.5 × 1.8 | 4.80 | 12 | 9.52 | 15.88 |
| 6.3 × 1.8 | 5.61 | 2 | 9.52 | 19.05 |
Key difference vs. phenol formaldehyde: Thermoplastics like acrylic and styrene resins have slightly more ductility, so installation holes can be marginally smaller. Note the tighter hole sizes here compared to the thermoset column above.
Clearance Holes: Passing a BF/BT Screw Through a Top Panel
When a BF or BT screw must pass through an outer panel before threading into the base material, you need a clearance hole — not a pilot hole.
Per ANSI/ASME B18.6.5M-1986, three clearance grades are defined. Normal clearance is preferred in most applications.
| Nominal Size × Pitch | Close Clearance (mm) | Normal Clearance — Preferred (mm) | Loose Clearance (mm) |
|---|---|---|---|
| 2.2 × 0.8 | 2.40 | 2.60 | 2.80 |
| 2.9 × 1 | 3.10 | 3.30 | 3.50 |
| 3.5 × 1.3 | 3.70 | 3.90 | 4.20 |
| 4.2 × 1.4 | 4.50 | 4.70 | 5.00 |
| 4.8 × 1.6 | 5.10 | 5.30 | 5.60 |
| 5.5 × 1.8 | 5.90 | 6.10 | 6.50 |
| 6.3 × 1.8 | 6.70 | 6.90 | 7.30 |
| 8 × 2.1 | 8.40 | 9.00 | 10.00 |
| 9.5 × 2.1 | 10.00 | 10.50 | 11.50 |
Clearance grade selection guide:
- Close clearance: Critical alignment requirements, thin wall sections, or where a minimal hole is needed. Countersinking/counterboring at the entry side may be necessary for proper head seating.
- Normal clearance (preferred): Standard assemblies where alignment is not critical and adequate adjustment is not needed.
- Loose clearance: Applications requiring maximum positional adjustment between assembled components.
Tolerance note: Values shown are minimum limits. Recommended plus tolerances are:
- Holes 1.70–5.80 mm diameter: +0.12 mm (close), +0.20 mm (normal), +0.30 mm (loose)
- Holes 5.80–14.50 mm diameter: +0.18 mm (close), +0.30 mm (normal), +0.45 mm (loose)
Inch-Series BF and BT: The Imperial Equivalents
While the metric data above follows ANSI/ASME B18.6.5M-1986, BF and BT screws also exist in inch sizes under ANSI B18.6.4-1981 (R1991).
The key structural difference from the metric description is spelled out explicitly in the inch standard:
The flute on Type BT screws has an included angle of 90 to 95 degrees and the thread cutting edge is located above the axis of the screw. Flutes and slots extend through first full form thread beyond taper except for Type BF screw on which tapered threads may be complete at manufacturer's option and flutes may be one pitch short of first full form thread.
Inch BF and BT — Thread and Point Dimensions (All dimensions in inches)
Threads share the same pitch series as Type B. The width of flat at thread crest shall not exceed 0.004 inch for sizes up to No. 8 inclusive, and 0.006 inch for larger sizes.
| Nominal Size | Basic Screw Dia. | Threads/Inch | D Major Max | D Major Min | d Minor Max | d Minor Min | P Point Dia. Max | P Point Dia. Min | S Taper Length Max | S Taper Length Min | Min L (90° Heads) | Min L (Csk Heads) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0.0600 | 48 | 0.060 | 0.054 | 0.036 | 0.033 | 0.031 | 0.027 | 0.042 | 0.031 | 1/8 | 1/8 |
| 1 | 0.0730 | 42 | 0.075 | 0.069 | 0.049 | 0.046 | 0.044 | 0.040 | 0.048 | 0.036 | 1/8 | 5/32 |
| 2 | 0.0860 | 32 | 0.088 | 0.082 | 0.064 | 0.060 | 0.058 | 0.054 | 0.062 | 0.047 | 5/32 | 3/16 |
| 3 | 0.0990 | 28 | 0.101 | 0.095 | 0.075 | 0.071 | 0.068 | 0.063 | 0.071 | 0.054 | 3/16 | 7/32 |
| 4 | 0.1120 | 24 | 0.114 | 0.108 | 0.086 | 0.082 | 0.079 | 0.074 | 0.083 | 0.063 | 3/16 | 1/4 |
| 5 | 0.1250 | 20 | 0.130 | 0.123 | 0.094 | 0.090 | 0.087 | 0.082 | 0.100 | 0.075 | 7/32 | 9/32 |
| 6 | 0.1380 | 20 | 0.139 | 0.132 | 0.104 | 0.099 | 0.095 | 0.089 | 0.100 | 0.075 | 1/4 | 9/32 |
| 8 | 0.1640 | 18 | 0.166 | 0.159 | 0.122 | 0.116 | 0.112 | 0.106 | 0.111 | 0.083 | 9/32 | 11/32 |
| 10 | 0.1900 | 16 | 0.189 | 0.182 | 0.141 | 0.135 | 0.130 | 0.123 | 0.125 | 0.094 | 5/16 | 3/8 |
| 12 | 0.2160 | 14 | 0.215 | 0.208 | 0.164 | 0.157 | 0.152 | 0.145 | 0.143 | 0.107 | 11/32 | 7/16 |
| 1/4 | 0.2500 | 14 | 0.246 | 0.237 | 0.192 | 0.185 | 0.179 | 0.171 | 0.143 | 0.107 | 3/8 | 1/2 |
| 5/16 | 0.3125 | 12 | 0.315 | 0.306 | 0.244 | 0.236 | 0.230 | 0.222 | 0.167 | 0.125 | 15/32 | 19/32 |
| 3/8 | 0.3750 | 12 | 0.380 | 0.371 | 0.309 | 0.299 | 0.293 | 0.285 | 0.167 | 0.125 | 17/32 | 11/16 |
| 7/16 | 0.4375 | 10 | 0.440 | 0.429 | 0.359 | 0.349 | 0.343 | 0.335 | 0.200 | 0.150 | 5/8 | 25/32 |
| 1/2 | 0.5000 | 10 | 0.504 | 0.493 | 0.423 | 0.413 | 0.407 | 0.399 | 0.200 | 0.150 | 11/16 | 27/32 |
Inch BF/BT Hole Sizes in Die Cast Zinc and Aluminum (All dimensions in inches)
| Screw Size | Thickness | Hole Size | Drill Size |
|---|---|---|---|
| 2 | 0.060 | 0.073 | 49 |
| 2 | 0.083 | 0.073 | 49 |
| 2 | 0.109 | 0.076 | 48 |
| 2 | 0.125–0.140 | 0.076 | 48 |
| 4 | 0.109 | 0.098 | 40 |
| 4 | 0.125–0.250 | 0.100–0.102 | 39–38 |
| 6 | 0.125–0.312 | 0.120–0.125 | 31–1/8 |
| 8 | 0.125–0.312 | 0.149–0.152 | 25–24 |
| 10 | 0.125–0.375 | 0.166–0.172 | 19–11/64 |
| 12 | 0.125–0.375 | 0.191–0.196 | 11–9 |
| 1/4 | 0.125–0.375 | 0.221–0.228 | 2–1 |
| 5/16 | 0.125–0.375 | 0.281–0.290 | K–L |
| 3/8 | 0.125–0.375 | 0.344–0.348 | 11/32–S |
Inch BF/BT Hole Sizes in Plastics
Phenol Formaldehyde:
| Screw Size | Hole Size | Drill Size | Penetration Min | Penetration Max |
|---|---|---|---|---|
| 2 | 0.078 | 5/64 | 0.094 | 0.250 |
| 3 | 0.089 | 43 | 0.125 | 0.312 |
| 4 | 0.104 | 37 | 0.125 | 0.312 |
| 5 | 0.116 | 32 | 0.188 | 0.375 |
| 6 | 0.125 | 1/8 | 0.188 | 0.375 |
| 8 | 0.147 | 26 | 0.250 | 0.500 |
| 10 | 0.170 | 18 | 0.312 | 0.625 |
| 12 | 0.194 | 10 | 0.375 | 0.625 |
| 1/4 | 0.228 | 1 | 0.375 | 0.750 |
Cellulose Acetate, Cellulose Nitrate, Acrylic Resin, Styrene Resin:
| Screw Size | Hole Size | Drill Size | Penetration Min | Penetration Max |
|---|---|---|---|---|
| 2 | 0.076 | 48 | 0.094 | 0.250 |
| 3 | 0.089 | 43 | 0.125 | 0.312 |
| 4 | 0.100 | 39 | 0.125 | 0.312 |
| 5 | 0.113 | 33 | 0.188 | 0.375 |
| 6 | 0.120 | 31 | 0.188 | 0.375 |
| 8 | 0.144 | 27 | 0.250 | 0.500 |
| 10 | 0.166 | 19 | 0.312 | 0.625 |
| 12 | 0.189 | 12 | 0.375 | 0.625 |
| 1/4 | 0.221 | 2 | 0.375 | 0.750 |
Material and Heat Treatment: What Your Screws Are Made Of
Per ANSI/ASME B18.6.5M-1986:
Tapping screws are normally fabricated from carbon steel and are suitably processed to meet the performance and test requirements outlined in the standard. Tapping screws may also be made from corrosion resistant steel, Monel, brass, and aluminum alloys. The materials, properties, and performance characteristics applicable to such screws should be mutually agreed upon between the manufacturer and the purchaser.
In practice, this translates to the following standard material options:
| Material | Typical Applications | Notes |
|---|---|---|
| Carbon Steel (standard) | General purpose, indoor environments | Most common; must be specified with coating for corrosion protection |
| Corrosion-Resistant Steel | Marine, chemical, food processing environments | Higher cost; verify thread hardness for cutting performance |
| Monel | Highly corrosive environments; salt water | Special order; expensive |
| Brass | Electronics, decorative applications, non-magnetic requirements | Lower hardness; verify cut performance in hard plastics |
| Aluminum Alloy | Lightweight assemblies; aerospace; non-ferrous requirements | Verify hardness sufficient for cutting action |
Surface finish considerations for BF and BT types:
- Plain (uncoated) carbon steel: baseline performance; forms basis for all hole size tables
- Zinc plating, chromium plating, other commercial finishes: may slightly increase effective diameter; generally acceptable without hole adjustment
- Heavy coatings (hot-dip galvanizing, thick epoxy): May require larger installation holes; validate by assembly trial