Hex socket head cap screw(Nissei Seiko(Coarse Steel Unplated 20 X 170 Pitch 2.5

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This is a bolt with a cylindrical head featuring a hexagonal recess.
A hex key (Allen wrench) is used for tightening. Although classified as a bolt, it is most commonly used without a nut.
It is also referred to as a cap screw or socket screw. Unlike open-end spanners or wrenches used to tighten hex bolts, which require clearance space around the fastener, a hex key requires no such surrounding space.
Therefore, the use of hex socket head cap screws enables work in confined spaces and contributes to the miniaturization of machinery.
Their appearance has also led to increasing use in decorative applications.

• Hex socket head cap screw Nissei Seiko Coarse pitch
• Nominal diameter (d) M20
• Length under head (ℓ) 170.0
• Pitch (p) 2.5 Thread length (b) Reference 52
• Head diameter (dk) Max (Nominal) (1) 30 Max (2) 30.33 Min 29.67
• Shank root diameter (da) Max 22.4
• Body diameter (ds) Max (Nominal) 20 Min 19.67
• Head height (k) Max (Nominal) 20 Min 19.48
• Under-head fillet (r) Min 0.8
• Hex socket width across flats (S) (3) Nominal (Nominal) 17 Max (Column 1) 17.23 Max (Column 2) 17.23
• Hex socket depth (t) Min 10 Head chamfer (v) Max 2 Full thread range 70
• Note: (1) Max without knurling
• (2) Max with knurling (Knurled unless otherwise specified)
• (3) Column 1: Applies to strength class 12.9. Column 2: Applies to strength class 10.9 and below, and stainless steel.