Wrench size

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spanner (wrench) size and thread diameter of a hex nut

Width across flats is the distance between two parallel surfaces on the head of a screw or bolt, or a nut, mostly for torque transmission by positive locking.

The term width across flats (AF) is used for the following forms:

• 2-socket = round material with two surfaces
• 4-socket = profile square section
• 6-socket, 8-square = regular polygons

Spanner

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The width across flats indicates the nominal "size" of the spanner. It is imprinted on the spanners in millimeter (mm) values. Older British and current US spanners (wrenches) have inch sizes that are imprinted in intermediate sizes in fractions.

The two systems are in general not compatible, which can result in rounding of nuts and bolts (i.e. using a 13 mm spanner in place of a 1&#;2 in (12.7 mm)). There are some exceptions with a few sizes being close enough to interchange. This includes 19 mm and 3&#;4 inch (19.05 mm), which are interchangeable for most purposes. Sizes that may interchange, depending on the precision needed, include 2 mm (close to 5&#;64 inch (1.98 mm)), 4 mm (close to 5&#;32 inch (3.97 mm)) and 8 mm (close to 5&#;16 inch (7.94 mm)).

In reality, a wrench with a width across the flats of exactly 15 mm would fit too tightly to use on a bolt with a width across the flats of 15 mm. The tolerances necessary to make the tools usable are listed in documents like ASME/ANSI B18.2.2 for U.S. standards.[1] For instance, a bolt for a 1-inch nominal diameter thread might have flats that are 1.5 inches apart. The wrench for this bolt should have flats that are between 1.508 and 1.520 inches apart to allow for a little extra space between the nut flats and the bolt flats. This means that the wrench slips easily over the bolt, and can be removed easily as well.

Width across flats

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Fastener terminology of hex nuts:
d: Nominal thread diameter
k: Wrench size (width across flats)
h: Thickness
e: Width across corners

e = 2 3 3 &#; k &#; 1. &#; k {\displaystyle e={\frac {2{\sqrt {3}}}{3}}\cdot k\approx 1.\cdot k}

The width across flats of the fastener (for example screws, nuts, clamps) is nominally the same as that on the tool. The table below shows dimensions of metric spanners for selected sizes of metric threads.[2] Note that with ISO 272 the width across flats for M10, M12, M14 and M22 were changed from 17, 19, 22 and 32 mm respectively to the current standard.

Nominal thread diameter (mm) M2 M4 M5 M6 M8 M10 M12 M16 M20 M24 M30 M36 M42 Width across flats (mm) 4 7 8 10 13 16 18 24 30 36 46 55 65

Widths for bicycles

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In addition to industry standards, there are special thread standards, such as the bicycle threads according to DIN , a fastening thread on metric-inch size basis of the designation. Modern bicycle spokes carry the bike thread FG 2.3.[3] A spoke wrench matching the spoke nipple is needed.

See also

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References

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[Editor's Note: This article originally appeared in the December issue of Grassroots Motorsports; for a torque wrench comparison from a later issue, click here.]

If you work on cars, you&#;re probably familiar with gutendtheit&#;we&#;ve all twisted wrenches until the bolt or nut felt &#;good and tight.&#; But is the bolt that&#;s keeping your engine together, suspension attached or wheel secured to your car really tightened to the proper torque? 

If it&#;s too low, the bolt or nut may work itself loose. If it&#;s too tight, you risk stripped threads or broken fasteners. If you&#;re like most of us, you probably use your trusty torque wrench when tightening an important fastener. After all, how well calibrated is your mind&#;s perception of gutendtheit, anyway?

Well, that raises another good question: How well-calibrated is your torque wrench?

While we know that a new, high-end torque wrench from Snap-On or Precision Instruments will be calibrated perfectly through its range of adjustment, once it lives in the trunk of your car or the bottom of your toolbox, all bets are off. We decided to do a simple calibration test on a trio of half-inch drive torque wrenches that lived in our workshop.

Our test was simple: Using a torque wrench calibration unit, we set each wrench to a specific torque value on its adjustor. Then we took readings of the value to which it actually tightened.. We took multiple readings on each wrench in 10 lb.-ft. increments, then averaged the result for a final value. Like most torque wrenches in use by GRM readers, none of our test wrenches had ever been serviced or calibrated since they were purchased.

The Test Subjects: Half-Inch Drive Torque Wrenches

• Harbor Freight click-style wrench, $16. We purchased this one brand new, and it hadn&#;t seen any use before our test. The manufacturer claims this wrench is accurate to +/-4 percent.
• 15-year-old Craftsman click-style wrench, $75. This trusty wrench had cycled through thousands of clicks, but we&#;d given it reasonable care and protection throughout its service life. According to Craftsman, this wrench is accurate to +/-4 percent.
• High-end split-beam wrench, $235. We purchased this one from Snap-On about three years ago. This mechanic-owned tool had been well cared for and never dropped. The manufacturer says it&#;s accurate to +/-4 percent.

The Results

Despite its wallet-friendly price, the brand-new Harbor Freight click wrench proved to be surprisingly accurate fresh out of the box. At $16, this unit becomes a disposable item should it fail or lose accuracy. It&#;s simply cheaper to buy a new one than to recalibrate it.

The accuracy of our $75 Craftsman store click wrench was a pleasant surprise after its many years of service. Its owner consistently set the wrench to the lowest torque setting when it wasn&#;t in use, which takes tension off the internal spring and helps prevent permanent damage that can affect accuracy.

Surprisingly, the expensive split-beam wrench from the tool company consistently over-torqued by almost 20 lb.-ft. from 80 lb.-ft. and above. This meant that every wheel lug nut we tightened was being over-torqued&#;and overstressed&#;by as much as 20 percent. On the upside, split-beam torque wrenches adjust very easily and don&#;t need to be placed at their lowest setting between uses. A few dollars spent to recalibrate this unit will bring it back to its original accuracy. 

Lessons Learned

Spending more doesn&#;t always guarantee better results. In the case of torque wrenches, however, we have a recommendation: Buy a long-lasting tool that can be recalibrated for a modest price if it drifts out of spec.

Another tip: Treat your torque wrench like the precision instrument it is. Don&#;t toss it in the bottom of your toolbox when you&#;re done using it.

A calibration check and adjustment will cost $30 to $40 from online providers. You can also ask your local tool truck man to check it, too. Click-type torque wrenches like the ones we tested will usually last for to cycles, or clicks. That&#;s more than tire changes, or about a penny a click for our mid-priced department store torque wrench. Recalibration means peace of mind&#;you&#;ll know it&#;s more than just gutendtheit.

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Comments

irish44j (Forum Supporter)

MegaDork

12/15/20 9:12 p.m.

Years ago, a Snap-On guy stopped at my neighbor's house to deliver a toolchest or something. I took the opportunity to have him test my Snap-On torque wrench (which at the time was a year old and only used for doing head bolts and other "precision" stuff), as well as a 6-7 year old Craftsman one that I kept in my track kit and had done thousands of lug nuts and always stayed at 90 lbft.

Been a while, but my recollection was that the Snap-On tested out about 4% off.

The Craftsman tested out at 3%, even though I never reset it, generally treated it pretty roughly, and it had a broken plastic locking collar. Sears replaced it a couple years later after I sheared off the nub while using it as an emergency breaker bar with a cheater pipe on the end of it, lol. 

RedT2

MegaDork

12/16/20 8:25 a.m.

If you are looking for more details, kindly visit Value Square Spanner.

High-end manufacturers would love for us to believe that their expensive products are superior to their lower-priced competitors.  And their marketing efforts do a good job of grooming us to think like suckers and willingly part with our hard-earned money.  The truth is, there is almost always a sweet spot where good value can be obtained for a reasonable price. 

Caveat emptor.

Curtis73 (Forum Supporter) GRM+ Member and MegaDork
12/16/20 8:33 a.m.

I have a Craftsman that I never had re-calibrated and it has torqued thousands of bolts.  When I started building the LS for the LeMans I figured I would have it tuned-up.  Turns out it was dead on with about 1% error at the top and bottom of the range.  I also have a really good one I picked up at an auction sale.  I forget the brand name, but the guy was a pro machinist and it was a name I recognized, like SK or CDI or Williams.  It looked unused and (although nearly dead-on at about 2%), it was not as accurate as the Craftsman.

Curtis73 (Forum Supporter) GRM+ Member and MegaDork
12/16/20 8:37 a.m.

RedT2 said:

High-end manufacturers would love for us to believe that their expensive products are superior to their lower-priced competitors.  And their marketing efforts do a good job of grooming us to think like suckers and willingly part with our hard-earned money.  The truth is, there is almost always a sweet spot where good value can be obtained for a reasonable price. 

Caveat emptor.

When I was a grease monkey everyone laughed at my Craftsman tools while they had Snap On or MAC.  Mine broke the same as theirs, but the difference was that I simply had to swing by Sears on my lunch break to get a new one.  They either had to wait a week for the driver to stop by or drive to whatever town they were in that day and hope he had one on the truck.

I learned quickly that they weren't paying a premium price for a better tool, they were paying for MLM.

mdshaw

Reader

12/16/20 9:18 a.m.

I know size matters for torque wrenches. I have this Armstrong 100-600 1" drive 41" long.  Got it from a renter that was short 1 month. Finally got to use it on some 250 lb/sqft torsion axle nuts.

GPz11 (Forum Supporter)

Reader

12/16/20 9:20 a.m.

Try swinging by a Sears now, they are all gone by me.

RossD

MegaDork

12/16/20 9:51 a.m.

In reply to GPz11 (Forum Supporter) :

Lowes carries craftmans now. Do they still take broken tools?

The0retical (Forum Supporter)

UberDork

12/16/20 10:33 a.m.

In reply to Curtis73 (Forum Supporter) :

I caught the same E36 M3 in A&P school. Right up until one of my classmates broke his square and it took 2 weeks to replace it.

That said I do own a few tools that are just better from Snap-on. The stubby ratcheting screwdriver and four-way angle head wrenches being chief among them.

For torque wrenches mid-grade or better. I've had a couple really bad experiences with the cheap Harbor Freight ones.

The best torque wrench, IMO, is one that is checked for calibration on a schedule and holds the calibration well.

Ben Jolly

New Reader

12/16/20 1:16 p.m.

RossD said:

In reply to GPz11 (Forum Supporter) :

Lowes carries craftmans now. Do they still take broken tools?

[Tongue click. Scoffs]

Yes. They do. Sadly my 'new' Craftsman ratchet is made in China and not as nice as the USA made one it replaced. But Lowes and Ace both honor the warranty to my knowledge. I need to get a screwdrive replaced so I'll go "audit" it once it get over being sick...

Ben Jolly

New Reader

12/16/20 1:27 p.m.

The0retical (Forum Supporter) said:

In reply to Curtis73 (Forum Supporter) :

I caught the same E36 M3 in A&P school. Right up until one of my classmates broke his square and it took 2 weeks to replace it.

That said I do own a few tools that are just better from Snap-on. The stubby ratcheting screwdriver and four-way angle head wrenches being chief among them.

For torque wrenches mid-grade or better. I've had a couple really bad experiences with the cheap Harbor Freight ones.

The best torque wrench, IMO, is one that is checked for calibration on a schedule and holds the calibration well.

The story so far: In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. -Douglas Adams

Another (formerly active) A&P here. That calibration and testing thing is pretty important. I used Craftsman torque wrenches exclusively and they were checked every year, had no problems using them during my professional career. Nowadays I use my own electric strain gauge to check all my torque wrenches every few months. The Craftsman torque wrenches still live in my box in the garage, and I carry one of the HF el cheapos in each of my cars. The HF wrenches are always in in spec, one of them I have had for over a decade, it has torqued God only knows how many lugs and it is still always in spec when I test it. I got it second hand from a pawnshop for like $5 too, no idea how old it actually is.

 

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