№ 000C: The Right Screw for Mounting Life-Safety Equipment

Common wood screw

Common wood screw. CC-BY-SA Wikimedia Commons.

In building my kit of metric measuring and installation tools, I set about finding the right type of screw for mounting the life safety equipment (fire extinguishers, smoke alarms, etc.).

And ran smack into the Invisible Metric Embargo, again.

Situation: Most Screws in Your Toolbox are Wrong for This

There is no ISO standard for wood screws, similar to ANSI/ASME B. 18.6.1, which uses the Medieval English Units we call U.S. Customary Units. There is a series of DIN standards, but such screws are impossible to find in the U.S. And apparently, the specification uses both metric diameters and gauge numbers, indicating it was modeled on an earlier non-metric standard.

Choosing the right screw for this purpose seems simple, but it’s not. Most of the screws you have lying around aren’t at all right.

Common screw-head types
Credit: CC-BY-SA by Sakurambo~commonswiki

Since we’re drilling into wood studs, it needs to have a wood screw thread, like that specified by ANSI/ASME B18.6.1. But a standard wood screw has a flat (countersunk, “e” at right) head, that does not lay flat against the plastic or metal mounting holes in the equipment brackets or mounting plates. And drywall screws are even worse, because their bugle head (it flares out with an arc that looks like the horn on a trumpet) weakens the screw and is likely to break if installed with too much torque. Either head style will deform the mounting holes, weakening the bracket.

We want a flat bearing surface on the bottom of the screw head. So you may have some of these in your parts bin, but they almost certainly have metal-cutting threads, not wood-cutting threads, so we’re not going to use those, either.

What we’re looking for is a washer-head screw, which has a flat bottom surface that is also much larger in diameter than a wood or drywall screw. These are typically called cabinet screws, because they are ideal for mounting heavy cabinets to wood studs. A pan head screw would be an option, but all of these have metal cutting threads, not wood cutting threads, and aren’t often long enough to get a solid grip in the stud or hollow-wall mounting anchor.

Since we’re going to be pre-drilling holes, but I’d like the screws to work well even when you haven’t drilled a pilot hole, we also want a self-drilling point meant for wood—a Type 17 point.

Obstacles: Bad Screw Drives and Too Many Sizes

Finally, it’s time to ditch the old and always-troublesome screw drives. Slotted screws have only two contact points, can’t be driven with an electric driver without the tip walking out, and are easily damaged by most screwdrivers.

Phillips screws are evil.

Phillips screws are evil.

A Philips head screw is only slightly better. It’s easy to center the bit and start driving, but it has a major flaw—the design of the cross geometry causes the bit to cam out of the screw as soon as there is any appreciable torque on the screw, so you have to push down on the driver to maintain the ability to turn it, and even then it’s likely to slip and cam out, damaging the screw and often the bit itself in the process. Pozidriv is a little better, but looks so similar to Philips that most people think it is Philips, but work worse than Philips when you try to drive them with a Philips screwdriver. And most people (and most hardware stores) don’t have Pozidriv screwdrivers or bits. (IMHO, they are one of the most evil tools ever invented.)

Then there is this funky thing that looks like Phillips, but isn’t. It’s a Japanese JIS Type s. Both the screw heads and screwdrivers look almost identical to Phillips, but are just different enough that the two are incompatible. Most JIS Type S screws have a single dot indented into the head, between two legs of the cross. If you have one of these screws on equipment you need to assemble or repair, and don’t have a JIS Type S screwdriver, you should go buy a set of JIS screwdrivers, and never touch the screw with a Phillips head.

This confusion over the cross-point type screwdriver standards (there are a slew of others I haven’t even mentioned!) is not just academic. I recently bought a Joie MSC International Egghead Pancake Pan, and after the first use, the handle suddenly became loose (from the heat, expanding the two materials at different rates). The handle plastic handle is attached to the metal pan with what looked like a Phillips screw, but when I tried a #2 Phillips head screwdriver, it didn’t really fit, and I couldn’t turn the screw. It was too big to use a #1 head (I tried), and a #3 Phillips didn’t even come close to fitting. There is no dot indent on this screw, so there is no obvious indication what type it is, but it sure as heck is not a Phillips head screw. (And no, I don’t use “Brand X” screwdrivers; my trusty old #2 Phillips is a Craftsman.)

And if you say Robertson FTW, we’re going to have to have a little talk about some other odd things you do up north, like slapping see-through km/h stickers with the mph number design instead of the proper one (and thus making visiting American drivers think the limit is miles per hour), and calling it done. Chain-yanking aside, Robertson screws don’t cam out like Philips, but they still only have 4 engagement points, and don’t support as high a torque load as do more modern driver designs. Just remember, not all square-drive screws are Robertson drive. You’ve got the same problem Phillips screws have (just not as many variations on the theme).

Torx screwdriver tip

Torx screwdriver tip. CC-BY-SA Wikimedia Commons

The only sane alternative is the Torx drive. Several things make this ideal. First, it’s got 6 points where the bit engages the screw (instead of 4 with Philips or Robertson), those 6 points exert a force on the screw nearly perpendicular the the path of rotation, and you can get nifty screwdrivers from Wiha that magically hold the screw to the tip of the driver like it was glued on.

Lastly, we come to size. The large number of screw sizes used in the U.S. is just nuts. There are 25 different sized wood screws listed in ANSI/ASME B18.6.1. We’re going to simplify that, and introduce the concept of preferred numbers.

This is a huge problem—the large number of screw sizes cascades and causes problems elsewhere. The more sizes of screws, the more sizes of drivers you need. The more sizes of machine screws, the more sizes of taps and dies (thread forming tools) you need (plus another pair of each for each size whenever that size has more than one thread density). And because you need several different diameters of holes for each screw size (clearance hole and two sizes of pilot holes for wood screws, one each for soft and hard woods; close fit, loose fit and clearance fit for machine screws). And for each screw/thread combination, you need to stock multiple lengths. And for each machine screw diameter and thread pitch combination, you need to stock different…


The Metric Maven introduced me to the concept in his essay Preferred Numbers and the “Preferred Measurement System”, and it sparked a whole bunch of ideas for me. We’ll eventually use one of those schemes to design a metric set of measuring cups and spoons, but right now, we’re going to use it to pick a screw size.

Actions: Torx and Preferred Numbers = Great Fasteners

We’re going to start with the ISO 262 Preferred Thread Sizes standard, which has 34 different screw sizes! Even just limiting the list to those found in the 1st choice column of Table 1 (a slightly adjusted R10 series) there are 21 sizes (37 if you count all of the coarse/fine thread combinations). This is still too many, so we’re going to start with the 21 sizes, eliminate all but the coarse threads, and then filter it with the Renard Series from ISO 497. More specifically, we’re using the partial series R”5 (2.5 … 25) (read: R double-prime five) to further constrain the selection. (Since there is no 15 mm screw or 25 mm screw in the first preferred series of ISO 262, the 15 is adjusted to 16, and the 25 is adjusted to 24.) Now we have just 6 screw/bolt sizes:

ISO 262 preferred sizes: M1, M1.2, M1.6, M2, M2.5, M3, M4, M5, M6, M8, M12, M16, M20, M24, M30, M36, M42, M48, M56, M64.

The first 5 of the 6 metric screws in the 1st preferred series.

GoMetricUSA preferred sizes: M2.5, M4, M6, M10, M16, & M24

Note that this does not quite (or obviously mesh with the R”5 series because it’s been filtered by ISO 261 and ISO 262 and all of the fudging that went on in those committees to come up with an acceptable international standard. But it can be argued it’s close enough in principle, even if the precise values aren’t immediately recognizable. (Also, note that the American interpretation of what ISO 262 does, ANSI B4.2 (Table 11.2), uses a slightly adjusted R10 series that is more faithful to the Renard series than is ISO 262.)

The above scheme was developed by Knut O Kverneland (author of World Metric Standards for Engineering and creator of GOmetricUSA.org) circa 1978.

But frustratingly, when trying to find these screws at my local Orchard Supply Hardware, it was impossible to collect the whole set. They simply do not stock M24 screws or bolts. And the smallest, the M2.5 screw, is only stocked in a slotted head!! I was able to find the M4 and M6 in Phillips drive, but not Torx drive. Above that, everything was hex-head or Allen-key bolts. No internal or external Torx. I’ll come back to this problem the first time I write about something that needs machine screws or bolts. For now, they have served the purpose of illustrating what the diameters look like.

I’m going to punt on the engineering calculations for now, and assume that a cabinet mounting screw will have sufficient strength to mount things that weigh less than a kitchen cabinet. Those screws are typically U.S. #8 gauge, and vary between 1 inch and 3 1/8 inches in length. For our purposes, we are going to avoid going too deep, thereby reducing the chances the drill or screw will strike another fastener on the other side of the wall, and shoot for one of the shorter lengths of 1 or 1 1/4 inches. Doing the USCS to metric conversion, we get a screw diameter of 4.064 mm, and a length of 31.75 mm. (This is fiendishly close to a DIN 96 M4.0x30 wood screw.)

Results: 6 Screw Sizes and One Drive Type

So here are all of the properties we’re looking for:

  • M4x30
  • Wood screw threads
  • Washer head
  • Torx drive
  • Type 17 self-drilling point
GRK cabinet screw

GRK cabinet screw

I was amazed to find exactly that in the GRK Fasteners Cabinet Screw. It’s officially a #8×1 1/4-inch  screw, but heck, they also describe the screw as being a 4 mm diameter, and 30 mm long. You can also see them in action on YouTube.

Finally, we’re going to drill pilot holes before installing screws in wood, because the one thing that most decreases pullout strength in wood screws is over-torquing the screw, and drilling the optimum size pilot hole will both reduce the torque needed to install the screw and provide it with the maximum pullout strength. The literature most commonly states that a pilot hole 85% (ideal) but no more than 90% the diameter of the screws minor (root) diameter is the ideal size. This is the diameter of the screw where the threads are, not including the threads. Based on the technical drawing, the GRK screws I’ve selected have a minor diameter of 0.106 inches. Convert that to metric and we get 2.6924 mm. Multiply that value by 0.85, and we need a 2.28854 mm pilot hole. Rounding up, the closest standard metric drill bit is 2.3 mm.

If we were drilling a bore hole for tapping metal, then we would need to use ISO 2857:1973, Ground thread taps for ISO metric threads of tolerances 4H to 8H and 4G to 6G coarse and fine pitches — Manufacturing tolerances on the threaded portion, but that’s another post for another topic. (I am researching the right set of taps, tap drill bits, and dies to match the six main GoMetricUSA preferred sizes.)

So now we have our ideal fire extinguisher bracket mounting screw.

Next up, we’re going to build a toolkit of the basic tools you need to stat creating a modern geek kitchen—metric measuring tools and a few other odds and ends.

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