Tap Bolts

Hex Tap Bolts: Fully Threaded Fasteners for Maximum Thread Engagement

Hex tap bolts, also known as fully threaded hex bolts or hex cap screws threaded to the head, represent a specialized category of hexagonal head fasteners designed for applications requiring full thread engagement along the entire shank length. Unlike standard hex bolts that feature an unthreaded grip length beneath the head, tap bolts are threaded from the bearing surface of the head to the tip, providing maximum versatility in tapped hole applications and variable-thickness joint configurations.

Design Characteristics and Specifications

The defining feature of hex tap bolts is their full-length threading, which extends from directly beneath the hexagonal head to the chamfered point. This design allows the fastener to engage threads immediately upon entering a tapped hole, regardless of material thickness or hole depth. The hexagonal head conforms to ASME B18.2.1 dimensional standards for hex cap screws, featuring a washer face bearing surface on the underside that provides consistent contact area and prevents head embedment in softer materials.

Thread forms follow ASME B1.1 specifications for Unified threads, with UNC (Unified National Coarse) being most common for general-purpose applications and UNF (Unified National Fine) specified where vibration resistance and fine adjustment are priorities. Metric hex tap bolts conform to ISO 4017 (DIN 933 equivalent) standards, featuring threads per ISO 68-1 and tolerance classes per ISO 965-1. The Class 2A thread fit for inch-series bolts and 6g tolerance for metric variants ensure reliable interchangeability with standard nuts and tapped holes.

Material Grades and Mechanical Properties

Hex tap bolts are manufactured across a comprehensive range of material grades to address diverse strength requirements and environmental conditions:

Grade 2 steel represents the baseline for low to medium-strength applications. With a minimum tensile strength of 74,000 PSI (510 MPa) for sizes up to 3/4" diameter and proportionally lower values for larger diameters, Grade 2 tap bolts suit non-critical structural connections, equipment mounting, and general assembly work where cost efficiency takes priority over high-strength performance.

Grade 5 steel delivers medium-strength performance with minimum tensile strength of 120,000 PSI (827 MPa) for sizes up to 1" diameter. Heat-treated and tempered, Grade 5 tap bolts feature three radial lines on the head for identification. This grade dominates automotive, agricultural equipment, and machinery applications where strength matters but the premium cost of Grade 8 isn't justified.

Grade 8 steel provides the highest strength in common commercial grades, with minimum tensile strength of 150,000 PSI (1034 MPa). Six radial lines identify Grade 8 fasteners. These tap bolts serve critical applications in heavy equipment, high-stress machinery, and safety-critical assemblies where failure consequences are severe.

Stainless steel tap bolts in 18-8 (304), 316, and 410 grades combine corrosion resistance with structural capability. Type 18-8 provides excellent general corrosion resistance and is the default choice for most outdoor and wet environments. Type 316 adds molybdenum for superior resistance to chlorides and marine atmospheres. Type 410 martensitic stainless allows heat treatment for higher strength but offers reduced corrosion protection compared to austenitic grades.

Silicon bronze tap bolts (alloy 651) provide exceptional corrosion resistance in marine environments while maintaining good strength (tensile strength approximately 70,000 PSI). Their non-sparking properties make them suitable for hazardous locations where flammable vapors may be present.

Advantages Over Standard Hex Bolts

The fully threaded design of tap bolts offers specific advantages that standard partially-threaded hex bolts cannot match. First, tap bolts accommodate variable material thicknesses without requiring precise length selection. A single tap bolt size can secure joints ranging from minimum thread engagement depth to nearly the full bolt length, simplifying inventory management for maintenance operations and multi-product manufacturing.

Second, in tapped hole applications where the bolt threads directly into a component without a nut, tap bolts maximize available thread engagement. This is particularly critical in soft materials like aluminum, cast iron, and plastics where thread stripping is a concern. Every available millimeter of thread engagement contributes to joint strength.

Third, tap bolts simplify blind hole applications where bolt length must be carefully matched to hole depth. The continuous threading means there's no unthreaded shank section that could bottom out against incomplete threads at the base of the hole.

Applications and Industry Uses

Tap bolts find extensive application in machinery assembly and maintenance, where access limitations often require bolting into tapped holes rather than through-holes with nuts. Engine blocks, gearbox housings, pump bodies, and valve assemblies commonly use tap bolts for cover plates, brackets, and accessory mounting.

Structural steel connections occasionally specify tap bolts where through-bolting is impractical or where one face of the joint is inaccessible for nut installation. This includes retrofit applications, equipment anchoring to embedded plates, and connections to hollow structural sections (HSS).

Furniture and fixture manufacturing utilizes tap bolts extensively for assembly into threaded inserts, tapped wood blocks, and cross-dowel connectors. The full threading allows adjustment during assembly and accommodates manufacturing tolerances in component positioning.

Installation Considerations

Proper installation of hex tap bolts requires attention to thread engagement depth, torque specifications, and hole preparation. Minimum thread engagement should equal 1.0 to 1.5 times the nominal bolt diameter in steel, increasing to 2.0D or greater in aluminum and other soft metals. Tapped holes should be deep enough to accommodate the intended bolt length plus chip clearance at the bottom.

Torque specifications for tap bolts follow standard values for the applicable grade and diameter, but the effective clamping length (unthreaded grip) is zero. This means the bolt stretches along threaded length during tensioning, which can affect fatigue performance in dynamic loading applications. For such applications, consider specifying partially-threaded hex bolts with grip length positioned in the shear plane.

Our selection of hex tap bolts encompasses all standard grades, materials, and sizes, with full-length threading ready to address your tapped hole fastening requirements. From light-duty Grade 2 to high-strength Grade 8, from corrosion-resistant stainless to marine-grade bronze, we stock the tap bolts your applications demand.