M3 x 5mm Flat Head T15 Torx Stainless Bolt (qty x10)

Ten M3 x 5 mm flat head screws in stainless steel, with a T15 Torx drive, Blue Alpine part F00114-00, for $2.50 a pack. Flat head and countersunk mean the same thing here: an M3 countersunk screw has a cone-shaped head that sinks into a matching recess and finishes level with the surface.

Where the 5 mm is measured on an M3 countersunk screw

On a countersunk head, length is the full figure from the flat top of the head to the tip, because the head ends up inside the part. So these are 5 mm in total. That’s short. Part of the 5 mm is the cone of the head, which leaves only a few millimeters of actual thread.

When you check an old one, put the caliper jaws on the top face of the head and on the point. Measuring from under the head, the way you would on a pan head, gives a smaller number and sends you looking for a size that doesn’t exist.

Is 5 mm enough thread?

M3 coarse has a 0.5 mm pitch, so every millimeter of engagement is two turns. A common rule of thumb is at least one screw diameter of engagement in steel, about 3 mm for M3, and more in softer metal like aluminum. With an M3 countersunk screw this short, the top part has to be thin and the thread it goes into has to sit right behind it. A press-in M3 nut or a tapped plate are the typical partners.

Longer isn’t automatically better. In a blind hole, an overlong screw bottoms out before the head seats. It feels tight, but the head is still proud and nothing is actually clamped.

T15, not T10 or T20

These take a T15 Torx bit. The six-lobe recess grips well at this size, but only with the correct bit. A T10 sits loose and rounds the lobes. A T20 won’t go in at all. Our T20 Torx screwdriver fits the T20 screws we sell, not this M3 countersunk screw, so you’ll need a T15 from your own kit.

Small recesses are shallow. Push the bit all the way in before turning, keep the driver in line with the screw, and retire a bit once its corners look rounded. A worn one cams out and chews the head, and a 5 mm stainless screw with a stripped recess is miserable to remove. The Torx entry on Wikipedia lists the full size range.

Countersink angle for an M3 countersunk screw

Metric countersunk heads have a 90-degree included angle. Inch flat heads use 82 degrees. Put a metric head into a hole cut with an 82-degree tool and it touches only around its outer rim, so the clamping load sits on a thin ring instead of the whole cone. Cut metric countersinks at 90 degrees and the head beds down evenly.

Snugging it down

Run the M3 countersunk screw in by hand until the head meets the cone, then finish with the driver. Stop when the head is flush and firm. A tapered head wedges as it seats, so it reaches tight sooner than you’d expect, and extra torque on a thread this small mostly goes into stretching or stripping it. Stainless into stainless can also gall if you drive it fast. Keep the speed down.

In the pack

• 10 x M3 x 5 mm flat head (countersunk) screws
• T15 Torx drive
• Stainless steel
• M3 x 0.5 metric coarse thread
• Blue Alpine part F00114-00

Stainless is the right call on small hardware in a damp machine. A rusted M3 countersunk screw sitting flush in a recess gives you nothing to grip once the drive is gone. If you’re not certain this is the screw your manual calls for, send the part number through our contact page or phone +1 (443) 514-5708 and we’ll compare it. The rest of the small hardware is in fasteners and tools.