Hex Head Cap Screws, UNC (ASME B18.2.1) are specified where the joint requires inch hex cap screws with UNC threads for general machinery. The sections below cover the typical end-uses by industry, the size and material decisions for each, assembly practice, and the applications this fastener type is not suited to. Stocked across roughly 1 to 7, they cover general to heavy-duty work.
Typical Applications for Hex Head Cap Screws, UNC
The most common settings where these are specified:
- Heavy equipment: US-built machinery.
- Automotive: inch fasteners on US vehicles.
- Agricultural equipment: North American machinery.
- Industrial maintenance: inch repairs.
How to Specify Hex Head Cap Screws, UNC for Your Application
- Size: Match the nominal size to the mating thread or hole. This product spans 1–7; check the full table below for the exact dimensions of each size.
- Strength class: Choose the property class (e.g. 8.8, 10.9, 12.9 or A2/A4 stainless) for the load and environment, and tighten to the matching torque so the joint relies on preload, not friction.
- Environment: For damp, coastal or chemical exposure prefer A4/316 stainless or a suitable coating; indoors, plated steel or A2 is usually sufficient.
- Standard: This product is supplied to ASME B18.2.1. Quoting the standard on your order guarantees interchangeable dimensions between suppliers.
Installation & Assembly Practice
- Tighten to the torque for the size and property class with a calibrated wrench — the joint is held by preload, not by the thread friction alone.
- Ensure at least one full thread projects beyond the nut after tightening, and never re-use a bolt that has been tightened into yield.
- Under slotted or oversized holes, fit a hardened washer so the preload does not relax into the hole edge.
Common Selection Mistakes with Hex Head Cap Screws, UNC
- Specifying class 12.9 "to be safe" in a corrosive environment — high classes are more sensitive to hydrogen embrittlement and stress corrosion; 8.8 hot-dip galvanized often outlasts them outdoors.
- Choosing a length that puts the shear plane through the threaded portion when a plain shank (partially threaded bolt) was available.
- Mixing stainless bolts with plated-steel nuts: the joint loses its corrosion logic and can gall or seize.
Where Hex Head Cap Screws, UNC Are Not the Right Choice
Not for permanent joints that will never be opened (a rivet or weld is cheaper), and not where the head must sit flush — use a countersunk screw instead.
Custom manufacturing · Made to drawing
Need a hex head cap screws, UNC outside the standard range?
Head form and drive can be specified to the assembly rather than to a standard: a larger bearing face, a security drive, or a socket sized for the tool already on the bench. Made by cold heading or hot forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
