🔩 M4 M5 M6 Countersunk Machine Screws – 316 Stainless Steel, Phillips Flat Head for Marine & High‑Corrosion Environments
You’re installing hardware on a boat, repairing chemical plant equipment, or building outdoor furniture near the ocean – and you need screws that won’t fail after one season. These M4, M5, and M6 countersunk machine screws are made from 316 stainless steel (marine grade). The flat, countersunk head sinks flush into a pre‑drilled hole, leaving a smooth, snag‑free surface. The Phillips drive gives you reliable engagement with standard crosshead bits. Choose from M4, M5, or M6 diameters with lengths measured overall including the head. 316 stainless contains molybdenum, offering superior resistance to saltwater, chlorides, and industrial chemicals. Perfect for marine rigging, docks, coastal architecture, food processing equipment, and any application where A2 (304) just isn’t enough.
✨ Key Features – 316 Stainless Steel Countersunk Phillips Screws
- 🔽 Countersunk flat head (90°) – Sits flush with the surface after proper countersinking. Ideal for smooth, professional finishes on wood, metal, and composite materials.
- 🛡️ 316 stainless steel (marine grade) – Contains molybdenum for exceptional resistance to saltwater, chlorides, acids, and industrial chemicals. Outperforms A2 (304) in harsh environments.
- 🌊
Phillips drive – Standard cross recess works with common screwdrivers and bits. Good torque control with reduced cam‑out.
- 📏 M4, M5, M6 metric threads – Popular medium sizes for marine hardware, machinery, furniture, and construction.
- 📐 Overall length includes head – Length measured from top of head to tip. Fully threaded for most lengths (longer screws may have partial thread).
- 💪 High strength & toughness – 316 stainless offers excellent tensile strength, impact resistance, and durability in extreme conditions.
- 🌊 Widely used – Marine equipment, docks, chemical plants, food processing, outdoor furniture, coastal architecture.
📏 Specifications – M4 M5 M6 Countersunk Phillips Screws (316 Stainless)
| Thread Diameter |
Available Lengths (mm, overall) * |
Thread Pitch (mm) |
Head Style |
Drive |
Material |
| 📐 Overall length = from top of head to tip (countersunk screws are measured this way) |
| M4 |
8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60 |
0.7 |
Countersunk / Flat
|
Phillips
|
316 Stainless (A4)
|
| M5 |
8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80 |
0.8 |
| M6 |
8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80 |
1.0 |
* Lengths shown are typical. Exact availability varies by size – select from dropdown. Most screws are fully threaded for lengths up to ~40mm; longer screws may have partial threads. Please allow minor manual measurement errors (±0.5mm).
📋 Why 316 Stainless Steel? (A4 vs. A2/304)
| Property |
316 Stainless (A4) |
304 Stainless (A2) |
| Corrosion resistance |
Excellent – resists saltwater, chlorides, acids, and chemicals (contains 2‑3% Mo) |
Good – not recommended for continuous saltwater exposure |
| Typical applications |
Marine, coastal, chemical plants, food processing, medical devices |
General indoor/outdoor, furniture, automotive trim |
| Strength (tensile) |
~700 N/mm² (A4‑70) |
~700 N/mm² (A2‑70) – similar |
| Magnetic |
Almost non‑magnetic (slightly after cold working) |
Almost non‑magnetic |
| Price |
Higher – premium marine grade |
Lower – general purpose |
❓ Frequently Asked Questions – 316 Stainless Countersunk Screws
1. Do I need to countersink the hole for these screws to sit flush?
Yes – these are true countersunk screws with a 90° cone head. For the head to be perfectly flush (or slightly below) the surface, you must drill a matching 90° countersink hole. Use a countersink bit of the appropriate size. In soft materials like wood or plastic, you can sometimes tap the head in, but proper countersinking gives a professional finish.
2. How is the length measured for countersunk screws?
For countersunk (flat head) screws, the length is measured overall – from the top of the head to the tip. So an “M5 x 20mm” screw has a total length of 20mm including the head. This is different from pan head or button head screws, which are usually measured under the head.
3. Are these screws fully threaded? What does “including head length” mean?
Most screws up to around 30‑40mm are fully threaded. Longer screws may have a partial thread (an unthreaded shank near the head) – which is standard for many countersunk screws. “Including head length” means the dimension given (e.g., 20mm) is the total from top of head to tip. If you need fully threaded for a specific longer length, please contact us.
4. Can I use these screws in saltwater? Are they truly marine grade?
Yes – 316 stainless is widely recognized as marine grade. It resists pitting and crevice corrosion in saltwater much better than 304. For continuous underwater immersion or high‑chloride environments (e.g., tropical seawater), 316 is the correct choice. For occasional splashing, 304 may suffice, but for long‑term reliability, go with 316.
5. What Phillips screwdriver size do I need for M4, M5, M6?
Use a #1 Phillips for M4, #2 for M5 and M6. Always match the bit size to the screw head to avoid cam‑out and damage. A #2 Phillips is the most common and works well for M5 and M6.
6. Can I use these screws with a nut or do they need a tapped hole?
Both – these are machine screws. They can be threaded into pre‑tapped holes of the same metric size (e.g., M4 tapped hole for M4 screw) or used with a matching metric nut. They are not self‑tapping. For wood or plastic, drill a pilot hole and use a threaded insert or a nut on the back.
