🔩 4#40 Countersunk Machine Screws – Choose Phillips or Hex Drive, Stainless or Carbon Steel, Nickel/Black Zinc/Natural
You’re working on a small electronic device, repairing a piece of furniture, or assembling a model – and you need a #4‑40 screw that sits flush. These countersunk machine screws are available with either a Phillips cross drive or a hex socket (Allen) drive, so you can pick the tool you prefer. Choose from 304 stainless steel for corrosion resistance or carbon steel (C1018) with nickel or black zinc plating. The flat 82° head sinks flush into a pre‑drilled countersink, leaving a smooth surface. Fully threaded, with overall length measured including the head. Available in lengths 5/16″, 3/8″, 0.4″, and 0.47″. Perfect for electronics enclosures, machinery, furniture, and DIY where a flush finish and reliable fastening are needed.
✨ Key Features – 4#40 Countersunk Machine Screws
- 🔽 Countersunk flat head (82°) – Sits flush with the surface after proper countersinking. Smooth, snag‑free finish.
- 💪
Two drive options – Choose Phillips (cross recess) or Hex socket (Allen) to suit your tool preference.
- 🛡️ Material choices – 304 stainless steel (natural, corrosion‑resistant) or carbon steel C1018 with nickel or black zinc plating.
- 📏 #4‑40 UNC thread – Standard US coarse thread size (4 gauge, 40 threads per inch). Widely used in electronics and small assemblies.
- 📐 Overall length includes head – Example: 4#40 x 5/16″ means total length from top of head to tip is 5/16″. Fully threaded.
- 💪 Strong & durable – Good mechanical properties, wear resistance, and corrosion protection (depending on material/plating).
- 🏭 Widely used – Furniture manufacturing, machinery assembly, electronic equipment, automotive, aerospace, medical devices, and shipbuilding.
📏 Available Sizes – 4#40 Countersunk Screws (Overall Length)
| Thread Size |
Overall Length (inches, including head) |
Head Diameter (approx.) |
Drive Options |
Material & Finish Options |
| 📐 Length = from top of head to tip. Fully threaded. Select exact size, drive, material and finish from dropdown. |
| 4#40 |
5/16" (0.3125") |
0.18″ or 0.23″ (varies by length – see dropdown) |
Phillips or Hex socket (Allen) |
304 Stainless (natural) / Carbon steel (nickel plated or black zinc) |
| 4#40 |
3/8" (0.375") |
| 4#40 |
0.4" (approx. 13/32") |
| 4#40 |
0.47" (approx. 15/32") |
* Please allow minor tolerances (±0.002‑0.005″). Head diameter varies with length – refer to product dropdown for exact specifications.
📋 How to Choose the Right Option
| Material / Finish |
Corrosion Resistance |
Best For |
| 304 Stainless (natural) |
Excellent – indoor/outdoor, humidity, light marine |
Outdoor furniture, marine electronics, medical devices |
| Carbon steel + nickel plated |
Good – indoor use, light moisture |
Electronics, appliances, visible fasteners |
| Carbon steel + black zinc |
Moderate – indoor, low‑glare finish |
Industrial equipment, dark assemblies |
Drive type: Phillips works with common crosshead screwdrivers; Hex socket (Allen) allows higher torque with matching hex key.
❓ Frequently Asked Questions – 4#40 Countersunk Screws
1. How do I measure the length of a countersunk screw?
For countersunk screws, length is measured overall – from the top of the head to the tip. So a “4#40 x 3/8″” screw has a total length of 3/8″ including the head.
2. Do I need to countersink the hole?
Yes – these are true 82° countersunk screws (UNC standard). Drill a matching 82° countersink hole for a flush finish. Use a countersink bit slightly larger than the head diameter.
3. Are these screws fully threaded?
Yes – all 4#40 screws in this range are fully threaded from under the head to the tip, giving you maximum grip in thin materials or when using nuts.
4. What tool do I need for hex drive versions?
Hex socket (Allen) drive screws require a 0.050″ or 1/16″ hex key (exact size varies – check product specs). Phillips versions use a #0 or #1 Phillips screwdriver.
5. Will carbon steel screws rust outdoors?
Nickel or black zinc plating provides moderate protection for indoor and light outdoor use. For continuous outdoor or marine environments, choose 304 stainless steel.
6. Can I use these screws with a nut? What size nut fits?
Yes – these are machine screws. Use a standard #4‑40 UNC hex nut. They will also thread into pre‑tapped #4‑40 holes.
