M3/M4 5mm to 20mm Flat Head Knurled Thumb Screws 303 Stainless Steel Hand Bolts

Measure the total thickness of the material being fastened (panel + any spacer or nut depth). The shank length should be long enough to engage at least 1.5× the thread diameter of full thread engagement in the mating nut or threaded insert. For a 2 mm aluminum panel with a standard M3 nut, a 5–8 mm shank is typically sufficient. For thicker panels or stacked materials, choose 10–16 mm.

These thumb screws are designed for hand-tightened, non-structural fastening — think access panels, inspection covers, and removable enclosure sides. 303 stainless steel has good tensile strength, but because they are tightened by hand without torque tools, clamping force is lower than a tool-driven fastener. Do not use them in load-bearing or vibration-critical structural joints.

303 stainless steel offers moderate corrosion resistance suitable for dry indoor environments such as PC cases, electronics enclosures, and workshop jigs. It is not recommended for outdoor exposure, high-humidity environments, or contact with saltwater or harsh chemicals. For outdoor or marine applications, 316 stainless steel would be a more appropriate choice.

M3 screws have a 3 mm thread diameter and are paired with an 11 mm head diameter. M4 screws have a 4 mm thread diameter and are paired with a 12 mm head diameter. M3 is common in compact electronics, small PC cases, and lightweight panels. M4 is used where slightly more clamping force or a larger head bearing surface is needed, such as thicker aluminum panels or heavier enclosure doors.

Yes, with the appropriate T-nut inserted into the extrusion slot. Select the thread diameter (M3 or M4) that matches your T-nut, then choose a shank length that accounts for the T-nut engagement depth plus any panel thickness. These are well-suited for frequently disassembled jig fixtures, enclosure doors on 3D printer frames, and modular workstation panels.

If you are prototyping or need a single replacement, the 1 or 2 PCS options let you test fit and confirm the size before committing. For a full PC build or enclosure project, 4–10 PCS typically covers all panel positions. For small-batch production, workshop stock, or kitting, the 25–100 PCS packs offer a lower per-unit cost. Check the variant pricing to find the pack size that best matches your project volume.

Both options work. These screws can thread into standard M3 or M4 tapped holes in metal panels, threaded inserts pressed into plastic or composite panels, or standard hex nuts and T-nuts. Ensure the mating thread is clean and the pitch matches the screw variant you select. Contact us to confirm thread pitch if you have a specific application requirement.