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.
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