1/4”-20 Grade 8 (Imperial thread)
The 1/4”-20 Grade 8 is a imperial (unified) threaded fastener with a 1.27 mm (20 threads per inch) thread pitch. It develops a maximum recommended assembly torque of 12 Nm dry, a single-shear capacity of 15 kN, and a minimum tensile strength of 1035 MPa. It is manufactured from quenched-and-tempered alloy steel with high strength and a black-oxide or zinc finish.
Fastener Specifications
| Specification | Value |
|---|---|
| Thread System | Imperial |
| Designation | 1/4”-20 |
| Thread Pitch | 1.27 mm (20 threads per inch) |
| Property Class / Grade | Grade 8 |
| Material | Alloy Steel |
| Tensile Strength | 1035 MPa |
| Max Assembly Torque | 12 Nm |
| Shear Strength (single) | 15 kN |
Thread & Grade Notes
1/4”-20 is a Imperial thread. Grade 8 fasteners are a high-strength grade with a minimum tensile strength of 1035 MPa. Thread pitch of 1.27 mm (20 threads per inch) determines how many turns are required to engage a given length and how resistant the joint is to loosening through vibration.
- Preload: torque to roughly 65–70% of proof load (≈ 12 Nm dry).
- Shear joints: a single bolt in shear carries about 15 kN before failure; use multiple fasteners or larger sizes for higher loads.
- Corrosion: matching fastener and structure materials avoids galvanic corrosion, especially with stainless in chloride environments.
Tightening & Substitution Notes
Always torque to the value specified by the joint engineer; the 12 Nm figure here is a generic dry assembly target. When substituting, match the thread system (Imperial), pitch (1.27 mm (20 threads per inch)), grade (Grade 8), and at minimum the shear capacity (15 kN). A lower-grade fastener in the same size reduces both preload and shear capacity; stainless grades trade some strength for corrosion resistance and are prone to galling.
Frequently Asked Questions
What is the thread pitch of the 1/4”-20?
1.27 mm (20 threads per inch).
How much torque does an 1/4”-20 Grade 8 bolt take?
Approximately 12 Nm dry assembly torque.
What is the shear strength of the 1/4”-20 Grade 8?
About 15 kN in single shear.
What is the tensile strength of the 1/4”-20 Grade 8?
1035 MPa minimum.
What material is the 1/4”-20 Grade 8?
Alloy Steel.