Hex Cap Screws
What Are Hex Cap Screws?
Hex cap screws are externally threaded fasteners with a six-sided head and a precision washer face under the head. They are designed to be tightened with a wrench and used with a nut or into a tapped hole. In common usage, “hex bolt” and “hex cap screw” are related terms; cap screws typically have tighter dimensional tolerances and a washer face for improved bearing under the head.
Fully vs. Partially Threaded Hex Cap Screws
- FT (Fully Threaded): Threads run the full length of the shank. Choose FT when the grip stack is thin or when full engagement through the joint is required (similar to a “tap bolt”).
- PT (Partially Threaded): A smooth shoulder (unthreaded section) sits under the head. PT is preferred when the shoulder will act as a bearing surface, for alignment, or when shear through the unthreaded section is desired.
Materials, Grades & Finishes for Hex Cap Screws
- Carbon/Alloy Steel (Inch Grades): Grade 2 (general purpose), Grade 5 (medium strength), Grade 8 (high strength). Typical finishes include zinc plated, yellow zinc, hot-dip galvanized (HDG), and black oxide.
- Stainless Steel: 18-8/304 for general corrosion resistance; 316 for marine or chemically aggressive environments. (A2 ≈ 304, A4 ≈ 316 in common metric notation.)
- Metric Property Classes: 8.8, 10.9, 12.9 indicate increasing strength levels. Most stock is zinc plated; stainless metric is commonly A2 or A4.
- Aluminum: Lightweight and corrosion resistant for non-structural applications where strength demands are modest.
- Hot-Dip Galvanized (HDG): Heavy zinc coating for outdoor exposure; use properly matched nuts to account for coating thickness.
Hex Cap Screw Sizes & Threads
Specify size as diameter × thread × length (e.g., 1/2"-13 × 3"). Inch threads are typically UNC (coarse) or UNF (fine). Coarse is most common and more forgiving in field assembly; fine is chosen for higher clamp load per turn and finer adjustment when the mating parts are compatible. Length is measured from under the head to the tip.
Common Uses for Hex Cap Screws
- Construction and steel fabrication (base plates, brackets, equipment mounting)
- Machinery/OEM assembly and maintenance
- Automotive and equipment repair
- Wood framing, jigs, and general fastening where a nut-and-bolt joint is preferred
Installation Tips for Hex Cap Screws
- Match nuts and washers to the bolt’s grade, material, and finish for consistent performance.
- Use proper torque or tensioning per project spec. For safety-critical joints, follow engineered procedures.
- Select coatings or stainless grades that suit the environment (interior, exterior, marine, chemical).
- For HDG fasteners, use compatible HDG nuts/washers to ensure correct thread fit.
- Consider thread-locking methods (prevailing-torque nuts, chemical threadlocker) where vibration is present.
Related Hardware for Hex Cap Screws
- Hex nuts (matching thread and grade), flange nuts, and prevailing-torque lock nuts
- Flat, fender, and lock washers; flange bolts (built-in washer) as an alternative
Hex Cap Screws FAQs
See the accordion below for differences vs. hex bolts, thread selection, grade markings, corrosion choices, and torque basics.
Information here is general guidance. Always follow the product’s datasheet and applicable codes.
Both are hex-head fasteners used with a nut or tapped hole. Cap screws typically have a washer face and tighter dimensional tolerances; “hex bolt” is a broader term used widely in construction and general fastening.
Use FT when the joint is thin or needs full thread engagement through the grip. Choose PT when you want a smooth shoulder for alignment/bearing or to carry shear through the unthreaded section.
Diameter = 1/2″, thread = 13 TPI (UNC coarse), length = 3″ measured from under the head to the tip.
UNC (coarse) is most common and more tolerant for field work. UNF (fine) is used when finer adjustment or higher clamp force per turn is desired and the mating parts support it.
Grade 2 (no radial marks), Grade 5 (three radial marks), Grade 8 (six radial marks). Higher grade = higher strength. Match nuts/washers accordingly.
They are strength designations for metric bolts. Higher numbers indicate higher strength. Pair with compatible metric nuts.
18-8 is a common trade term roughly equivalent to Type 304; A2 is the typical metric designation for a similar composition. For harsher environments, use 316 (metric A4).
Use HDG or stainless outdoors or where corrosion is a concern. HDG offers a heavy zinc coating; stainless resists corrosion without a coating. Choose based on environment, appearance, and cost.
Yes. Use properly matched HDG nuts (or nuts tapped to suit HDG threads) so the coating thickness doesn’t interfere with assembly.
Test123
Add the material stack thickness plus the desired projection of threads beyond the nut (commonly 1–3 threads after tightening).
The washer face under a cap screw improves bearing, but flat washers are still common to spread load and protect surfaces. Consider flange bolts if you want a built-in washer.
Torque depends on size, grade, finish, and lubrication. Use the product’s torque table or project specification. For structural or safety-critical joints, follow engineered procedures.
In non-critical applications they’re often reused if undamaged. For safety-critical joints or where preload accuracy matters, follow your project or manufacturer guidance.
Structural connections usually require heavy-hex structural bolts to the specified structural standard. Use the fastener type your engineer or code requires.