ASME/ANSI B18.3 Countersunk Head Torx Screw
ASME/ANSI B18.3 Countersunk Head Torx Screws are designed for applications requiring a low-profile, flush-mounted fastening point combined with controlled Torx drive engagement. The countersunk head seats into a matching countersunk hole so that the head remains at or below the surrounding surface, making the screw suitable for assemblies where protruding fastener heads could interfere with adjacent components.
The six-lobe Torx recess provides a positive driver interface, allowing installation torque to be transferred through multiple contact surfaces and reducing the tendency of the driver to slip from the recess during tightening. This is particularly useful when consistent installation torque is required across repeated assembly operations. For mechanical performance, alloy steel versions can be specified to ASTM F835 and stainless steel versions to ASTM F879, where applicable. The appropriate material and mechanical property requirement should be selected according to the required strength, corrosion resistance, installation conditions, and service environment.
Specification:
ASME/ANSI B18.3 defines the key dimensional requirements for countersunk head socket screws, including nominal size, length, head geometry, and thread dimensions. For this Torx version, the countersunk head is designed for flush seating in a prepared hole, while the six-lobe recess provides a positive interface with the installation tool. Mechanical properties depend on the selected material. Alloy steel versions can be specified to ASTM F835, while stainless steel versions can be specified to ASTM F879, where applicable. These specifications cover relevant properties such as tensile strength, yield strength, elongation, and hardness.
| Item | Specification |
|---|---|
| Standard | ASME/ANSI B18.3 |
| Product Type | Countersunk Head Screw |
| Head Style | Flat Countersunk Head |
| Drive Type | Torx/6-Lobe |
| Thread System | Inch |
| Thread Series | UNC/UNF |
| Material | Alloy Steel,Stainless Steel |
| Alloy Steel Standard | ASTM F835 |
| Stainless Steel Standard | ASTM F879 |
| Surface Treatment | Zinc Plated, Black Oxidation,Bright Finish,Passivation |
Dimension Chart of ASME/ANSI B18.3 Countersunk Head Torx Screw:
The dimension chart provides the key measurements required to match the countersunk screw with the mating component and Torx driver. In addition to the nominal diameter and screw length, the countersunk head dimensions and Torx recess size are important for proper installation and tool compatibility. The head diameter and head height determine how the screw seats into the countersunk hole, while the recess size must match the appropriate Torx driver. Customers should confirm the countersink geometry of the mating component together with the screw dimensions when designing a flush fastening connection.
| Thread Diameter | 2# | 4# | 5# | 6# | 8# | 10# | 1/4 | 5/16 | 3/8 | 7/16 | 1/2 | 5/8 | 3/4 | 7/8 | 1 | |
| d | 0.086 | 0.112 | 0.125 | 0.138 | 0.164 | 0.19 | 0.25 | 0.3125 | 0.375 | 0.4375 | 0.5 | 0.625 | 0.75 | 0.875 | 1 | |
| PP | UNC | 56 | 40 | 40 | 32 | 32 | 24 | 20 | 18 | 16 | 14 | 13 | 11 | 10 | 9 | 8 |
| UNF | 64 | 48 | 44 | 40 | 36 | 32 | 28 | 24 | 24 | 20 | 20 | 18 | 16 | 14 | 12 | |
| UNEF | - | - | - | - | - | - | 32 | 32 | 32 | 28 | 28 | 24 | 20 | 20 | 20 | |
| ds | max | 0.086 | 0.112 | 0.125 | 0.138 | 0.164 | 0.19 | 0.25 | 0.3125 | 0.375 | 0.4375 | 0.5 | 0.625 | 0.75 | 0.875 | 1 |
| min | 0.0822 | 0.1075 | 0.1202 | 0.1329 | 0.1585 | 0.184 | 0.2435 | 0.3053 | 0.3678 | 0.4294 | 0.4919 | 0.6163 | 0.7406 | 0.8647 | 0.9886 | |
| dk | max | 0.197 | 0.255 | 0.281 | 0.307 | 0.359 | 0.411 | 0.531 | 0.656 | 0.781 | 0.844 | 0.938 | 1.188 | 1.438 | 1.688 | 1.938 |
| min | 0.168 | 0.218 | 0.24 | 0.263 | 0.311 | 0.359 | 0.48 | 0.6 | 0.72 | 0.781 | 0.872 | 1.112 | 1.355 | 1.604 | 1.841 | |
| k | Ref | 0.064 | 0.083 | 0.09 | 0.097 | 0.112 | 0.127 | 0.161 | 0.198 | 0.234 | 0.234 | 0.251 | 0.324 | 0.396 | 0.468 | 0.54 |
| A | Ref | 0.094 | 0.111 | 0.111 | 0.132 | 0.155 | 0.178 | 0.221 | 0.266 | 0.312 | 0.352 | 0.352 | 0.446 | 0.529 | 0.619 | 0.795 |
| t | Ref | 0.03 | 0.035 | 0.04 | 0.04 | 0.045 | 0.05 | 0.065 | 0.087 | 0.105 | 0.1 | 0.119 | 0.135 | 0.205 | 0.265 | 0.285 |
| b | Ref | 0.62 | 0.75 | 0.75 | 0.75 | 0.88 | 0.88 | 1 | 1.12 | 1.25 | 1.38 | 1.5 | 1.75 | 2 | 2.25 | 2.5 |
| α | ±2° | 82° | 82° | 82° | 82° | 82° | 82° | 82° | 82° | 82° | 82° | 82° | 82° | 82° | 82° | 82° |
| Slot No. | T8 | T10 | T10 | T15 | T20 | T25 | T30 | T40 | T45 | T50 | T50 | T55 | T60 | T70 | T90 |
- RELATED SCREW LISTS:
- ANSI/ASME B18.3 Stainless Steel Hexagon Socket Button Head Cap Screws
- ANSI/ASME B18.3F Stainless Steel Hex Socket Countersunk Head Machine Screw
- American Standard ASME/ANSI B18.3 Hexagon Socket Head Cap Screws
- ASME/ANSI B18.6.3 Cross Recessed Pan Head Machine Screws
- DIN7991 Stainless Steel Hex Socket Countersunk Head Cap Screws
- ISO7380 Stainless Steel Hexagon Socket Button Head Screws
- DIN963 90° Stainless Steel Slotted Countersunk Head Screws
- DIN965 Stainless Steel Cross Recessed Countersunk Head Screws
- DIN84 Stainless Steel Slotted Cheese Head Screws
- DIN7971 Stainless Steel Slotted Pan Head Tapping Screws
- DIN7981 Cross Recessed Pan Head Tapping Screws
- DIN7972 Stainless Steel Slotted Countersunk Head Tapping Screws
Application of ASME/ANSI B18.3 Countersunk Head Torx Screws:
Flush-Mounted Machine Components
Countersunk Head Torx Screws are suitable for machine covers, guards, housings, mounting plates, and fabricated components where the screw head needs to remain flush with the surrounding surface. The recessed head reduces interference with adjacent or moving components.
Panels and Enclosures
They can be used for equipment panels, electrical enclosures, instrument housings, access covers, and control equipment where a smooth exterior surface is preferred. The Torx recess provides a positive drive interface for repeated installation and removal.
Compact Assemblies
The combination of a low-profile countersunk head and six-lobe Torx drive makes these screws useful in compact assemblies where both surface clearance and controlled tightening are important, including fixtures, brackets, precision equipment, and component mounting systems.