Machine Screws, Cap Screws and Set Screws: BS 4183 Machine Screw Nuts (Metric, Pressed Type)

BS 4183 Machine Screw Nuts (Metric, Pressed Type)

All dimensions in millimeters.

Square Nuts:

Hexagon Nuts:

British Unified Machine Screws: The Transatlantic Agreement

When Britain, America, and Canada Agreed on Screws

British Standard B.S. 1981:1953 represents a landmark in fastener standardization — it covers machine screws for which Britain, the United States, and Canada reached agreement on general dimensions for interchangeability. All screws under this standard use Unified threads.

Covered head types:

• 80° Countersunk (vs. 82° ANSI — note the 2° difference)
• 80° Raised Countersunk
• Pan Head
• Raised Cheese Head
• Hexagon Head

Identification Markings

All BS Unified machine screws must carry distinguishing features:

• Recessed heads: Groove in the form of four arcs of a circle in the upper head surface
• Hexagon heads: Circular recess in the upper surface, or continuous line of circles indented on one or more hex flats parallel to the screw axis, or at least two contiguous circles on the upper surface
• Slotted heads: Circular recess or circular platform/raised portion on the upper head surface
• Machine screw nuts (pressed type): Groove indented on one face approximately midway between thread major diameter and flats
• Machine screw nuts (precision type): Groove on one face or continuous line of circles on one or more hex flats

The Machine Screw Head Type Decision Matrix

the practitioner's Three Rules

Rule 1: Always start with the application requirement — flush, above-surface, or wrench-driven — and eliminate incompatible head types immediately.

Rule 2: Match the drive type to the assembly method. Manual assembly with flatblade? Slotted is fine. Power assembly on a production line? Cross recess (Type II or III) or hex is mandatory.

Rule 3: When in doubt between two head types, choose the one with the larger bearing surface. You can always reduce clamping area with a washer, but you can never increase it after the fact.

Clearance Holes for Metric Machine Screws

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

Proper clearance holes prevent thread interference and allow alignment adjustment during assembly.

Recommended Nominal Lengths in the supplied reference (Metric)

Not all lengths are available in all head types. The ANSI standard specifies which nominal screw lengths are recommended for each screw size and head style combination.

Key symbols:

• A = Screws of ALL head styles
• P = Pan head screws only
• H = Hex and hex flange head screws only

For lengths between the heavy lines in the standard tables, the appropriate head styles are available. As a general rule:

• Shorter screws are available in more head styles (pan and hex heads are available in shorter lengths)
• Countersunk heads require minimum lengths proportional to the head height
• Hex and hex flange heads are available in the longest length range

British Standard Thread and Shank Rules

Thread Lengths (BS 4183)

Minimum thread lengths for BS 4183 screws are:

Screws too short for the minimum thread length are threaded as close to the head as practicable. The unthreaded shank length does not exceed 1½ pitches for lengths up to twice the diameter, and 2 pitches for longer lengths.

Shank Diameter

The unthreaded shank diameter is not greater than the basic major diameter of the thread and not less than the minimum effective diameter. The actual diameter depends on manufacturing method:

• Turned screws: Shank diameter closer to the major diameter
• Cold-headed screws: Shank diameter closer to the effective (pitch) diameter

Radius Under the Head

For pan and cheese head screws, the radius under the head runs smoothly into the face of the head and shank without any step or discontinuity. A true radius is not essential provided the curve is smooth and lies wholly within the maximum radius.

Material Specifications

BS 4183 Minimum Tensile Strength Requirements

Common Material Designations (ANSI)

Machine screws are commonly specified in:

• Steel with zinc plating, nickel plating, or chromium plating
• Brass for electrical applications and corrosion resistance
• Corrosion Resistant Steel (stainless) for marine, food processing, and chemical environments
• Property Class 4.8 (metric) for standard-strength applications
• Property Class 9.8 or higher for high-strength applications (requires Style B head form for recessed countersunk screws)

The Universal Takeaway: What Every Engineer Must Remember

the practitioner's nightmare was preventable. The information existed. The standards were published. The dimensional tables were available. What was missing was a systematic approach to head type selection — the kind of framework that transforms a random hardware choice into an engineered decision.

Here is what you take from this guide:

1. The head type is not a cosmetic choice — it is a structural, functional, and economic decision. Every head profile distributes load differently, accepts torque differently, and interfaces with the assembly differently.

2. ANSI has formally deprecated two head types. The round head has been superseded by the pan head. The truss head is labeled "inherently weak" and not recommended for new designs. Stop specifying them.

3. Slotted hex heads are a manufacturing liability. The burrs from the secondary slotting operation interfere with wrench engagement. Specify plain hex or cross-recessed heads instead.

4. The 100-degree flat countersunk head is non-preferred. Unless your clearance analysis specifically demands it, use the standard 82° (ANSI) or 90° (BS) countersunk head.

5. Thread length is not optional — it is governed by screw size and nominal length. Verify your thread engagement against the standard minimums before finalizing screw length.

6. Cross recess Type III (square/Robertson) provides the highest torque transfer and zero cam-out of any recess type. If your assembly process and market permit it, this is the optimal drive system.

7. Always specify the complete designation. Nominal size, thread pitch, nominal length, head type, drive type, material, property class, and finish — in that order. Incomplete callouts create procurement ambiguity that costs time and money.

Your Next Step

Open your current project's bill of materials. Find every machine screw line item. For each one, ask yourself three questions:

1. Is this the right head type for the application? Use the decision matrix above.
2. Is the drive type optimized for the assembly method? Manual, power, or automated?
3. Is the thread length adequate for the joint thickness? Check against the standard minimums.

If any answer is "I'm not sure," you now have every table, every dimension, and every selection criterion you need to make it certain.

The cost of getting it wrong is measured in production rework. The cost of getting it right is measured in the ten minutes you just invested reading this guide.

Standards referenced: ANSI B18.6.3-1972 (R1991), ANSI/ASME B18.6.7M-1985, BS 4183:1967, BS 450:1958, B.S. 1981:1953. All dimensional data reproduced for engineering reference. Always verify against the current edition of the applicable standard for production specifications.

The Pulley That Nearly Killed a Career

The grinding noise came first. Then the smoke.

the practitioner, a third-year maintenance technician at a mid-size packaging plant, watched in horror as a 24-inch drive pulley walked off its shaft, shearing two belt guards and sending a 40-pound cast iron flywheel across the production floor. Nobody was hurt — this time. But the root cause report told a story that haunts machine shops everywhere:

A single undersized set screw, installed without understanding its holding power, had been expected to do the work of a properly engineered fastening system.

the practitioner had used a 1/4-inch cup-point set screw to lock a pulley transmitting 3 horsepower at 1,000 RPM on a 1-inch shaft. He picked the screw because it fit the tapped hole. No calculation. No reference to the holding power formulas. No consideration of point style, material grade, or shaft surface condition.

That incident became the practitioner's turning point — and it can become yours. Because cap screws and set screws are among the most misunderstood, most frequently misapplied fasteners in all of mechanical engineering. They look simple. They are not.

This guide is your permanent reference. Every dimension, every formula, every point style, every standard — extracted directly from ANSI/ASME B18.6.2-1998, ANSI/ASME B18.3-1998, and BS 4168 — organized so you can find exactly what you need, whether you are a first-year apprentice or a senior design engineer specifying fasteners for a critical assembly.

What You Will Find in This Guide

• Cap Screws — Slotted head styles (flat countersunk, round, fillister), hexagon and spline socket head, flat countersunk socket, button head socket, and shoulder screws
• Set Screws — Slotted headless, hexagon and spline socket, square head — with complete point style dimensions for cup, flat, cone, dog, half-dog, and oval points
• Optional Cup Point Variations — Seven manufacturer types (A through G) with full dimensional data
• Hexagon and Spline Socket Dimensions — The master socket reference table
• Keys and Bits Applicability — Which hex key fits which screw
• Holding Power Formulas — How to calculate whether your set screw can actually do the job
• British Standard (BS 4168) Metric Series — Cap screws, set screws, countersunk, and button head in metric dimensions
• Bright Square Head Set Screws (BSW/BSF) — The complete British Whitworth and Fine thread specification
• Worked Examples — Real calculations you can replicate on the shop floor

Cap Screws vs. Set Screws: The Critical Distinction

Before you reach for a fastener, you need absolute clarity on this point:

A cap screw is an externally threaded fastener that must be torqued by its head into a tapped or other preformed hole to perform its intended service. It has a head — flat countersunk, round, fillister, hexagon socket, button, or shoulder — and clamps two or more parts together by pulling them into contact through thread engagement.

A set screw is a headless (or square-headed) threaded fastener that is driven entirely into a tapped hole until the point bears against a mating surface — typically a shaft. Its purpose is to prevent relative motion (usually rotation) between two components such as a pulley, gear, collar, or coupling and the shaft on which they sit.

The failure mode is different. A cap screw that fails generally loses clamping force — the joint opens. A set screw that fails loses holding power — the component slips or walks off the shaft. the practitioner's pulley incident was a textbook set screw failure.

The rule: Cap screws hold parts together. Set screws hold parts in position.

Cap Screws — The Complete Dimensional Reference

Governing Standard

American National Standard ANSI/ASME B18.6.2-1998 covers slotted head cap screws and slotted headless set screws. ANSI/ASME B18.3-1998 covers hexagon and spline socket head cap screws, flat countersunk head cap screws, button head cap screws, shoulder screws, and socket set screws.

Thread Specification

Threads on slotted cap screws conform to the Unified Standard, Class 2A — UNC, UNF, and 8 UN Series (or UNRC, UNRF, and 8 UNR Series for rolled threads).

Threads on socket head cap screws conform to Class 3A UNRC and UNRF for sizes #0 through 1 inch, and Class 2A UNRC for larger sizes.

Length of Thread Formula

The length of complete (full form) thread on cap screws equals:

$$L_T = 2D + 0.250 \text{ in.}$$

With a plus tolerance of 0.188 in., or an amount equal to 2.5 times the pitch of the thread — whichever is greater. Cap screws too short to accommodate the minimum thread length have full form threads extending to within 2.5 pitches of the head.

Designation System

Slotted head cap screws are designated in this sequence:

Nominal size (fraction or decimal equivalent) → Threads per inch → Screw length (fraction or decimal equivalent) → Product name → Material → Protective finish (if required)

Examples:

• 1/2-13 × 3 Slotted Round Head Cap Screw, SAE Grade 2 Steel, Zinc Plated
• .750-16 × 2.25 Slotted Flat Countersunk Head Cap Screw, Corrosion Resistant Steel

Slotted Flat Countersunk Head Cap Screws (ANSI/ASME B18.6.2-1998)

These screws sit flush with the work surface when driven to full depth — ideal where protruding heads would interfere with moving parts or mating surfaces.

All dimensions in inches.

Slotted Fillister Head Cap Screws (ANSI/ASME B18.6.2-1998)

The fillister head provides the deepest slot of all slotted head styles, making it the preferred choice where high driving torque is needed and the head can protrude above the surface. The distinctive two-tiered profile (side height plus domed top) gives a total head height significantly greater than a round or flat head of the same nominal size.

Fillet Radius: Sizes 1/4 to 3/8 → .031 max, .016 min; sizes 7/16 to 9/16 → .047 max, .016 min; sizes 5/8 to 1 → .062 max, .031 min.

All dimensions in inches.

Hexagon and Spline Socket Head Cap Screws (ANSI/ASME B18.3-1998)

This is the workhorse of modern precision assembly. The socket head cap screw (often called a "socket head" or "Allen head" screw) delivers maximum clamping force in minimum space. Its cylindrical head allows installation in counterbored holes where hex bolt heads would not fit, and the internal hex drive transfers high torque without cam-out.

The 1960 Series dimensions below represent the current ANSI standard. Sizes range from #0 (0.060") all the way to 4" diameter.

All dimensions in inches. Key engagement depths are minimum values.

Drill and Counterbore Sizes for Socket Head Cap Screws (1960 Series)

This table is worth printing and taping to the wall next to your drill press. It tells you exactly what drill to use for the body clearance hole and what diameter to counterbore.

Close Fit: For assemblies with one screw or matched tooling. Normal Fit: For conventional tolerancing with multiple screws.

Countersink Note: It is considered good practice to countersink or break the edges of holes smaller than (D Max + 2F Max) in parts having a hardness approaching or exceeding the screw hardness. Without countersinking, screw heads may not seat properly and fillets may deform, making the screw susceptible to fatigue failure in dynamic loading applications.

Hexagon and Spline Socket Flat Countersunk Head Cap Screws (ANSI/ASME B18.3-1998)

When you need a flush surface AND internal hex drive capability, this is the fastener. Available in sizes #0 through 1-1/2 inch. Threads are Class 3A UNRC and UNRF for sizes #0 through 1 inch, Class 2A for larger sizes.

Standard length increments:

• 1/16" for lengths 1/8 through 1/4"
• 1/8" for lengths 1/4 through 1"
• 1/4" for lengths 1 through 3-1/2"
• 1/2" for lengths 3-1/2 through 7"
• 1" for lengths 7 through 10"

All dimensions in inches.

Socket Button Head Cap Screws (ANSI/ASME B18.3-1998)

Button head cap screws provide a low-profile, aesthetically pleasing head for light fastening applications. They are not recommended for critical high-strength applications where socket head cap screws should be used instead.

Standard length increments: 1/16" for lengths 1/8 through 1/4"; 1/8" for lengths 1/4 through 1"; 1/4" for lengths 1 through 2".

Length tolerances: −0.03" for lengths up to 1" inclusive; −0.04" for lengths 1 through 2".

All dimensions in inches.

Socket Head Shoulder Screws (ANSI/ASME B18.3-1998)

Shoulder screws (also called stripper bolts) have a precision-ground, enlarged unthreaded shoulder between the head and the thread. They serve as pivot pins, axles, and guide pins in tooling, fixtures, and mechanisms. The shoulder diameter is the nominal size; the thread is always smaller than the shoulder.

Standard length increments: 1/8" for lengths 1/4 through 3/4"; 1/4" for lengths above 3/4 through 5"; 1/2" for lengths over 5".

Thread class: Unified Standard Class 3A, UNC.

All dimensions in inches.