1/4”-20 18-8 (Imperial thread)
The 1/4”-20 18-8 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 8.5 Nm dry, a single-shear capacity of 11 kN, and a minimum tensile strength of 700 MPa. It is manufactured from austenitic A2 / 18-8 stainless steel with good corrosion resistance and non-magnetic behaviour.
Fastener Specifications
| Specification | Value |
|---|---|
| Thread System | Imperial |
| Designation | 1/4”-20 |
| Thread Pitch | 1.27 mm (20 threads per inch) |
| Property Class / Grade | 18-8 |
| Material | Stainless 304 |
| Tensile Strength | 700 MPa |
| Max Assembly Torque | 8.5 Nm |
| Shear Strength (single) | 11 kN |
Thread & Grade Notes
1/4”-20 is a Imperial thread. 18-8 fasteners are a medium-strength grade with a minimum tensile strength of 700 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 (≈ 8.5 Nm dry).
- Shear joints: a single bolt in shear carries about 11 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 8.5 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 (18-8), and at minimum the shear capacity (11 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 18-8 bolt take?
Approximately 8.5 Nm dry assembly torque.
What is the shear strength of the 1/4”-20 18-8?
About 11 kN in single shear.
What is the tensile strength of the 1/4”-20 18-8?
700 MPa minimum.
What material is the 1/4”-20 18-8?
Stainless 304.