- Upper: ¼ inch, 24 threads per inch
- Lower: 12mm or 0.477 inch diameter solid, cylindrical shaft
1/4-24 threading is non-standard, falling between standardized SAE 1/4-20 UNC and 1/4-28 UNF. It’s sometimes written “1/4-24 UNS”, with “S” for “Special”, but even then the dimensions and tolerances vary. You won’t find 1/4-24 fasteners in hardware stores, even in the USA.