Machine Screws, Cap Screws and Set Screws: Metric Hex and Hex Flange Head Dimensions

Metric Hex and Hex Flange Head Dimensions

Hex Head:

All dimensions in millimeters per ANSI/ASME B18.6.7M-1985.

Hex Flange Head:

All dimensions in millimeters per ANSI/ASME B18.6.7M-1985.

Important: M10 size screws having heads with 15 mm width across flats are not ISO standard. Unless M10 size screws with 15 mm width across flats are specifically ordered, M10 screws with 16 mm width across flats will be furnished.

Recommended Nominal Screw Lengths (Metric)

The following chart shows which screw lengths are recommended for which sizes. A signifies all head styles; P signifies pan head only; H signifies hex and hex flange head only.

Per ANSI/ASME B18.6.7M-1985. Lengths within recommended range are standard stock items.

Clearance Holes for Metric Machine Screws

Selecting the correct clearance hole is critical for assembly alignment and fastener performance.

Normal clearance is preferred. Values are minimum limits per ANSI/ASME B18.6.7M-1985 Appendix.

Recommended plus tolerances:

• Clearance hole diameters over 1.70 to 5.80 mm: +0.12 (close), +0.20 (normal), +0.30 mm (loose)
• Clearance hole diameters over 5.80 to 14.50 mm: +0.18 (close), +0.30 (normal), +0.45 mm (loose)

Header Points for Metric Machine Screws

Machine screws normally have plain sheared ends, but when specified, header points can be furnished:

Header points apply to nominal lengths shown or shorter. Longer lengths may require machining. All dimensions in mm per ANSI/ASME B18.6.7M-1985.

British Standard Machine Screws: The Metric Series (BS 4183:1967)

For engineers working with British-standard equipment or supplying components to UK-based operations, BS 4183 provides the metric machine screw framework. While marked as obsolescent (intended to eventually align with ISO standards), these specifications remain in widespread use for maintenance and replacement applications.

Material Properties

The materials from which BS 4183 screws and nuts are manufactured must meet minimum tensile strengths:

Understanding the 4.8 strength grade designation:

$$\text{First digit} = \frac{\text{Min. Tensile Strength (kgf/mm}^2\text{)}}{10} = \frac{40}{10} = 4$$

$$\text{Second digit} = \frac{1}{10} \times \frac{\text{Yield Stress}}{\text{Min. Tensile Strength}} \times 100 = \frac{1}{10} \times \frac{32}{40} \times 100 = 8$$

$$\text{Min. Yield Stress} = 4 \times 8 = 32 \text{ kgf/mm}^2$$

Thread Standard

BS 4183 screws use ISO metric coarse pitch series threads in accordance with BS 3643 Part 1. External threads conform to tolerance Class 6g (medium fit) and internal threads to Class 6H (medium fit).

Nominal Length Measurement

How you measure a machine screw's length depends on the head type:

BS 4183 Countersunk Head Dimensions

All dimensions in millimeters per BS 4183:1967.

BS 4183 Pan Head Dimensions

All dimensions in millimeters per BS 4183:1967.

BS 4183 Machine Screw Nuts (Pressed Type)

Square and Hexagon Nuts:

All dimensions in millimeters per BS 4183:1967.

British Unified Machine Screws (BS 1981:1953)

British Standard BS 1981:1953 covers machine screws and nuts where agreement has been reached with the United States and Canada for general dimensional interchangeability. All have Unified threads.

Covered Head Types

• 80° Countersunk head
• 80° Raised countersunk head
• Pan head
• Raised cheese head
• Small hexagon head
• Precision and pressed nuts (hexagon)

Unified Identification Markings

These screws and nuts carry distinguishing features to identify them as Unified:

• Recessed head screws: A groove in the form of four arcs of a circle on the upper head surface
• Hexagon head screws: One of three options—(1) circular recess on the head top, (2) continuous line of circles indented on one or more flats parallel to the axis, or (3) at least two contiguous circles on the head top
• Pressed-type nuts: A groove indented on one face approximately midway between the thread major diameter and the nut flats
• Slotted head screws: A circular recess or a raised circular platform on the head top

British Unified Countersunk Head Dimensions (80°)

All dimensions in inches per BS 1981:1953.

Thread Length Rule for British Unified Screws

• Unified countersunk and raised countersunk head screws: Screws 2 inches long and under are threaded right up to the head
• Other Unified, BSW, and BSF machine screws: Screws 2 inches long and under have an unthreaded shank equal to twice the pitch
• All screws longer than 2 inches: Minimum thread length of 1-3/4 inches

the practitioner's Transformation: The Systematic Selection Protocol

Six months after her 3 AM crisis, the practitioner's approach to machine screw specification had transformed completely. She developed a five-step protocol that eliminated screw selection errors across her entire design team:

Step 1: Define the Surface Condition

Flush required? → Flat countersunk (82° or 100°) or oval countersunk

Protruding acceptable? → Pan, hex, fillister, or truss head

Decorative finish needed? → Oval countersunk

Step 2: Determine the Drive Type

Manual assembly with screwdriver? → Slotted or cross recess (Type I/IA)

High-torque, cam-out sensitive? → Cross recess Type II (Pozidriv) or Type III (square)

Wrench or socket assembly? → Hex head

Step 3: Verify Thread Engagement

Calculate the minimum engaged thread length for your application. Ensure the screw length minus the unthreaded portion provides adequate engagement in the tapped hole or through the nut.

$$L_{engaged} = L_{screw} - L_{unthreaded} - t_{washer} - t_{clearance}$$

Where:

• $L_{screw}$ = Nominal screw length
• $L_{unthreaded}$ = Unthreaded body length (per thread length rules above)
• $t_{washer}$ = Washer thickness (if used)
• $t_{clearance}$ = Thickness of parts being clamped

Step 4: Check Clearance Geometry

Verify that the head diameter, head height, and slot depth don't interfere with adjacent components. This was the practitioner's original mistake. Pan head heights are significantly taller than flat countersunk heads for the same screw size.

Quick comparison for M4 screws:

Step 5: Specify Completely

Use the standard designation format. Never order a machine screw by size alone. Always specify:

1. Nominal size and thread (e.g., M4 × 0.7 or 1/4-20)
2. Nominal length
3. Head type and drive type
4. Material and property class
5. Protective finish (if required)

Complete specification example:

M6 × 1 × 25 Type IA Cross Recessed Pan Head Machine Screw, Class 4.8 Steel, Zinc Plated

The Standards Cross-Reference

Machine screw standards don't exist in isolation. Here's the complete cross-reference map:

Industry policy note: The British Standards Institution approved a 1965 policy statement urging British firms to regard Whitworth, B.A., and BSF threads as obsolescent, making ISO metric the first choice and ISO Unified (inch) the second choice for all future designs.

Engineering takeaway

When the practitioner finally received her correct M4 × 0.7 flat countersunk head screws three days later, she assembled the prototype in under an hour. The client deadline was met—barely. But the real lesson wasn't about the deadline.

The lesson was this: machine screw selection is an engineering decision, not a purchasing decision.

Every dimension in every table in this guide exists because someone, somewhere, needed that exact specification to make an assembly work. The tolerances exist because the difference between a 0.452-inch head diameter and a 0.507-inch head diameter is the difference between a screw that fits your countersink and one that sits proud by three thousandths.

Here's what to carry forward:

• Head type is geometry, not preference. Each head exists for a specific engineering function. Match the head to your functional requirement, not your familiarity.
• Thread length rules are screw-size-dependent. Don't assume a screw is threaded to the head. Check the thread length specification for your specific size and length combination.
• Metric and inch standards are not interchangeable by eye. An M4 screw and a #8 screw look similar in a catalog. They are not the same. Verify your system before ordering.
• Clearance holes have three tiers. Normal clearance is preferred. Close clearance restricts alignment adjustment. Loose clearance may not prevent bolt rotation.
• Complete designation prevents errors. Size, pitch, length, head type, drive type, material, class, and finish. Every field matters.

Your Next Step

Print this guide. Pin the head type decision matrix to your design station. The next time you reach for a machine screw, you won't be guessing—you'll be engineering.

If you found this reference valuable, consider these companion topics for your fastener engineering library:

• Torque-tension relationships for machine screws — understanding preload in small fastener applications
• Self-tapping screws vs. machine screws — when to thread-form and when to use a tapped hole
• Socket head cap screws — the high-strength alternative to slotted and recessed heads

What machine screw specification question has tripped you up in the past? The best engineering reference is the one that answers the question you're actually asking. Share your challenge—the answer might save someone else's 3 AM deadline.