M5 x 20mm Flat Head T20 Torx Stainless Bolt (qty x10)

This M5 x 20mm screw has a flat, countersunk head with a T20 Torx recess, and it’s stainless steel. A pack of ten is $3.00. Blue Alpine part number: F00121-00.

Measuring an M5 x 20mm screw

A flat head screw is measured from the top of the head to the tip, because the head ends up level with the surface. So all 20 mm counts, head included.

That makes the math easy when you bolt through. With the head sitting flush, the length sticking out the back is 20 mm minus the total thickness of everything you’re clamping. Say it passes through 2 mm of sheet and a 3 mm bracket. That leaves 15 mm behind the bracket, which is plenty of room for a washer and an M5 nut with thread to spare.

Going into a tapped hole instead of a nut, the same sum tells you how deep the hole has to be. An M5 x 20mm screw through a 3 mm countersunk plate puts about 17 mm of thread into the part below. If the hole is blind and shallower than that, the screw bottoms out before the head seats.

The thread is M5 x 0.8, so 5 mm in diameter and 0.8 mm per turn. Every millimeter you need to take up on a nut is a little over one turn.

A T20 recess on an M5 screw

Most M5 Torx heads take a T25. This one takes a T20, the same bit as our M4 Torx screws. That’s handy if you’re already holding a T20, but it means the recess is a size smaller than the thread would usually get.

A smaller recess can take less torque before the bit slips. The practical answer is to tighten the nut, not the screw. Hold the M5 x 20mm screw still with the T20 so the head doesn’t spin in its countersink, and turn the nut with an 8 mm wrench or nut driver. The wrench does the tightening, and friction between the head and its countersink helps stop it turning, so the bit works much less hard than it would driving the screw.

When you do have to drive the screw itself, into a tapped hole or a clinch nut, keep the bit fully seated and push in firmly. Replace the bit at the first sign of rounded lobes. A chewed T20 recess on a stainless screw is hard to get out.

Bolting an M5 x 20mm screw through, step by step

• Check the countersink. The head should drop in level with the surface, not sit proud.
• Push the screw through all the layers from the countersunk side.
• On the back, fit an M5 washer, then the nut. No washer goes under the countersunk head.
• Run the nut down by hand. Stainless on stainless can gall if you spin it on fast.
• Hold the head with the T20, snug the nut with the wrench and stop.
• Look at the end. A thread or two should show past the nut.

If a lot more than that sticks out, the M5 x 20mm screw is longer than the job needs, and the 12 mm version may suit better. A shorter screw usually looks neater than a cut one.

Choosing among the M5 flat heads

We carry three M5 countersunk screws, all stainless:

• M5 x 8mm, T25, for thin parts into a thread close behind.
• M5 x 12mm, T20, a middle length.
• This M5 x 20mm screw, T20, the one to reach through thicker stacks or out to a nut.

Mind the drive on the 8 mm one. It’s the odd one out and needs a T25 bit.

Material and help

Like our other M5 screws, the M5 x 20mm screw is stainless, so moisture that would rust plain steel doesn’t bother it. The trade-off is that stainless is prone to galling, which is why the go-slow advice keeps coming up. Wikipedia’s stainless steel page explains the alloy if you’re curious.

Not sure which length came out of your machine? Send us the part number or the measurement through our contact page, and we’ll match it. Everything else is under fasteners and tools.