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You’re mounting a deck board, installing a stainless steel hinge on a boat, or assembling outdoor furniture – and you need screws that sit flush and won’t rust. These M4 to M8 countersunk machine screws are made from 304 stainless steel, so they hold up in rain, humidity, and even coastal air. The flat, 90° head sinks perfectly into a pre‑drilled countersink, leaving a smooth, snag‑free surface. Phillips drive works with standard crosshead screwdrivers. Lengths are measured overall (including the head), and every screw is fully threaded for maximum grip. Available in M4, M5, M6, and M8 diameters. Pick the size you need from the dropdown – ideal for marine hardware, furniture, machinery, and outdoor DIY.
| Thread Diameter | Overall Length (mm, including head) * | Thread Pitch (mm) | Head Style | Drive | Material |
|---|---|---|---|---|---|
| 📐 Overall length = from top of head to tip. Fully threaded. Select exact size from dropdown. | |||||
| M4 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60 | 0.7 |
Countersunk / Flat |
Phillips |
304 Stainless Steel |
| M5 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80 | 0.8 | |||
| M6 | 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 | 1.0 | |||
| M8 | 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 | 1.25 | |||
* Lengths are typical – exact availability may vary. Please allow minor manual measurement errors. Not all lengths shown; select your required size from the dropdown.
| Property | Details |
|---|---|
| Material | 304 stainless steel (also known as 18-8) – contains 18% chromium, 8% nickel. Excellent corrosion resistance for outdoor and marine use. |
| Magnetic | 304 stainless is normally non‑magnetic, but cold working (like thread rolling) can create slight magnetism. This is normal and does not affect corrosion resistance. A strong magnet test is not a reliable indicator of quality. |
| Surface finish | Natural / untreated – raw stainless steel appearance. No added coating, so it maintains its corrosion resistance even if scratched. |
| Comparison | 304 is superior to 201 (which rusts easily) and is the standard for most outdoor applications. For continuous saltwater immersion, 316 stainless is recommended – but 304 is fine for decks, furniture, and marine above‑water use. |
1. How do I measure the length of these 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 standard for flat head fasteners.
2. Do I need to countersink the hole for these screws to sit flush?
Yes – these are true 90° countersunk screws. For the head to be perfectly flush or slightly recessed, you must drill a matching 90° countersink hole. Use a countersink bit of the appropriate size. In soft wood, you can sometimes tap the head in, but proper countersinking gives the best result.
3. Are these screws fully threaded? What about longer lengths like 100mm?
Yes – all M4 to M8 screws in this range, up to 100mm overall length, are fully threaded from under the head to the tip. This ensures maximum grip in both blind holes and through‑nut applications.
4. Can I use these screws outdoors or in a marine environment?
Absolutely. 304 stainless steel is highly corrosion‑resistant and is the standard choice for outdoor furniture, boat fittings (above waterline), decking, and coastal areas. For continuous saltwater immersion (e.g., underwater hardware), we recommend 316 stainless. But for 95% of outdoor DIY projects, 304 is perfect.
5. Will these screws rust if scratched?
Unlike plated carbon steel, 304 stainless has chromium that forms a self‑repairing oxide layer. Even if scratched, the fresh surface will quickly re‑passivate and resist rust. That’s why stainless is the go‑to for long‑term outdoor use.
6. Can I use these screws with a nut? What size nut fits?
Yes – these are machine screws. Use standard metric nuts of the same thread size (e.g., M6 nut for an M6 screw). They will also thread into pre‑tapped holes. They are not self‑tapping – for wood or plastic, drill a pilot hole and use a nut or threaded insert.
