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Nuts, bolts, screws, washers, andother mechanical fasteners holdnearly everything together, fromengines and transmissions, to riggingand hull-to-deck joints. For the aver-age boatbuilder, equipment installer,or repair yard, the importance ofchoosing the proper fastener for the jobcannot be over-emphasized. Anynumber of fasteners will often appear

to work for a given task; but, in mostcases, only one will do the job reliablyand safely. As a professional, theresponsibility is yours to make the cor-rect fastener selection and then toensure it is properly installed.

With that in mind, let’s review thekey elements of fastener technology.

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For machine screws, nuts, and bolts,it’s all in the torque and resulting

clamping force applied to the partsbeing secured. Whether that’s anengine cylinder-head or a mast-mounted VHF antenna, applying theproper torque to the appropriate boltor screw could very well mean thedifference between fastener successand failure.

For self-tapping screws, it is headdesign, not torque capacity or brutestrength, that often determines selec-tion.

Even if you’ve been building or repairing or installing (ordoing the purchasing) for years, we’re betting that certaincritical details about, and properties of, the fundamentalfasteners of the marine trades may well have gone overlooked.

Above—Not all fasteners are madeequal. Those that lack provenance, specific identifiable head markings,and/or a clear paper trail from manu-facturer to supplier may not perform as anticipated. Right—Several fasten-ers in this installation failed in a similarmanner, indicating a manufacturingdefect.

NNuuttss.. BBoollttss.. SSccrreewwss..

Text and photographs

by Steve D’Antonio

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Nuts and BoltsUnderstanding the components of a

common nut-and-bolt combinationwill aid in its proper use. The head,which a wrench fits around or driverfits into, comes in many configura-tions. The most popular is the hex, orsix-sided, variety. Head diameter, mea-sured from flat to flat across the cen-ter, is the wrench size, but not the fas-tener size (more on fastener measure-ment below).

The head is often stamped with var-ious markings that indicate its tensilestrength and/or chemical properties.Beneath the head is the shank, whichis, in some cases, an unthreaded sec-tion between the underside of thehead and the threaded section. Thetransition between the head and theshank is usually radiused to promotestrength, while the transition betweenthe shank and the thread—referred toas the runout—is rounded for thesame reason. It’s important to notethat not all fasteners possess theseimportant radii features, which aretypically found in rolled, as opposedto cut, threads.

Though casual inspection may

reveal little difference among fasten-ers, under closer scrutiny some boltsfail to measure up. This is particularlytrue for so-called “field-cut” fasteners.Those are bolts made somewhereother than in a fastener factory—oftenin the boatyard itself or in a nearbyshop. Regardless of the skill of the dieor lathe operator, and even assumingthe machining tools are well adjustedand maintained, the process of cuttingthreads into round or bolt stock invari-ably does so across the grain structureof the metal, imparting an interruptedflow of stress through the bolt.

I realize what I’ve just said may beheresy to machinists and tool-and-diemakers everywhere, but it’s not to saythreads can’t be cut. Rather, it’s simplythat the metallurgy behind the processmeans rolled threads are stronger andmore fatigue resistant when like alloysand dimensions are directly com-pared. Cutting threads leaves behindtorn, irregular surfaces, which makefor stress risers; hence, rolled fastenerthreads are preferable to those cut bya die or lathe. Rolled threads arenothing new (they’ve been aroundsince the 1870s); then and now they

Far left—While they are not necessarilya guarantor of quality or performance,head markings are useful. Fastenersbearing the F593C designation shouldmeet specified standards. Left—Correct fastener, wrong length. Thebolts in this fin stabilizer installationwere fine until the fin moved throughthe extremity of its arc and shearedthem. Bolt specification includes sev-eral factors; diameter and length areamong the most important.

Above left—Machine screws are available in a variety of thread types;the most obvious distinction is that offine (left) or coarse (right) threads.Above right—Rolling (rather than cut-ting) is the preferred method for makingmachine-screw or “parallel” threads.Rolled threads are often easy to iden-tify by the transitional step between thethreads and the shank.

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are less expensive to manufacture.Nearly all commercially availablemachine-thread fasteners are rolled.

There are different rolling pro-cesses and related protocols. Theyinclude: heat treatment before andafter rolling; employing reciprocatingversus cylindrical dies; and two versusthree dies. Suffice it to say, morecostly fasteners often use more prefer-able manufacturing processes.

The nominal length of an ordinaryhex bolt is measured from the under-side of the head to the opposite endof the fastener. The grip length, onthe other hand, is measured betweenthe underside of the head and theclosest parallel surface of a nut thathas been threaded onto the bolt whileallowing two threads to stand proudfrom the fastener’s end.

As a rule of thumb, a properlysized bolt when torqued should allowtwo threads to protrude from the nut’snon-pressure-bearing end. This isespecially important for nuts thatemploy embedded locking devices(such as plastic inserts) or intentionaldistortion of the final threads. Therule ensures that the nut has adequatethread engagement to the bolt to pre-vent thread collapse or stripping.

ScrewsGenerally speaking, a screw can

either be a bolt—hex head or other-wise—without its mating nut, or aself-tapping fastener, such as a woodscrew. The term screw in this regardis interchangeable.

The primary difference between abolt and a tapping screw is thethreads. Bolts and machine screws

employ a parallel thread, which mustbe mated with a nut, or screwed intoa threaded hole. Tapping screws,whose threads are tapered rather thanparallel, make their own threadedhole as they are turned into compara-tively soft material such as wood,sheet metal, fiberglass, or aluminum.(Properly sized pilot holes must bedrilled in the harder materials.)

Above—Fasteners with a locking mechanism, such as these nylon insertnuts in a shaft coupling, are commonlyinstalled on moving and vibratingmachinery. But the locking mechanismis effective only if engaged by the bolt’sthreads. When a nut and bolt is assem-bled, two threads should stand proud ofthe nut, with or without a locking mechanism.

Above—Fasteners in highly loaded applications fully exposed to the weather mustbe chosen carefully. Find the correct alloy—typically 18-8 or, where greater corro-sion resistance is required, 316—as well as the proper dimension. To minimizemovement and chafe, fasteners must engage all threads in the base structure, and be of the same diameter as the hole in the hardware they secure.

Left—The most widely available stainless steel washer is comparatively thin andnot suited for heavy machinery. The torque load of the fastener alone has distortedthe washer on this motor mount. More substantial washers often must be orderedfrom fastener specialty warehouses rather than typical marine chandlers. Right—Lag bolts, though common in engine-mount applications, are not ideal. A through-bolt is preferred for heavy machinery, particularly when it’s cyclically loaded. Manylag bolts are plated carbon steel, which corrodes rapidly in substrates prone tomoisture saturation.

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Because many tapping screws aredesigned for sheet metal, they arefully threaded and lack the smoothshank section often found on woodscrews.

The variety of these features makesfor a nearly endless selection of com-binations. The number of differenttapping, wood, and machine screwtypes found aboard a modest cruisingvessel, from the hull and deck toengineroom and interior joinerwork,could easily number in the scores ifnot hundreds.

Before moving on, let’s briefly dis-cuss the lag screw or bolt.Terminology gets a bit fuzzy here; thisfastener is part bolt thanks to its hex

head, and part screw by virtue of itsaggressive tapping threads. Lag boltsare typically designed for severe ser-vice in substrates softer than metal,such as wood and fiberglass. Theirmost common application aboardsmaller vessels is to secure an enginemount to its bed or stringer. I’ve seenmany of these fasteners fail in thisapplication because of rust. Stringersand engine beds often become satu-rated over time, and the threads on thelag bolt simply disintegrate. Eventually,under a heavy engine load—such ashard backing, or driving up a waveunder power—engine torque begins tolift the bolts out of the stringer on theuncompressed side.

Older vessels with lagged motormounts are almost certain to possessmild-steel lags. In this scenario, thehead of the fastener may appearsound, while the threads have actu-ally rusted away. The problem hasbeen mitigated with the increasedavailability and popularity of stainlesssteel lag bolts. But even with a stain-less fastener, if the stringer substrate,often wood, becomes sodden anddecays, then the fastener may pullout.

Today, many production and cus-tom builders have moved away fromlag bolts—stainless or otherwise—forengine security. They opt instead forthrough-bolts and motor mountshelves, laminated-in-place studs, orpre-drilled and tapped stainless oraluminum plates that accept conven-tional hex-head stainless bolts.

If you encounter an engine securedwith mild-steel lag bolts, they shouldbe removed and inspected for signsof rust or saturated stringers. Bothconditions need immediate correction.

Head styles for tapping andmachine screws include the commonpan head, which looks like anupside-down frying pan; oval and flatheads, which are self-explanatory; tothe less common fillister and cheeseheads, which are simply tall, and talltapered, heads, respectively.

Pan heads work fine where sur-face clearance and appearance aren’tan issue; or where they won’t bebunged. Flat heads offer zero surfaceclearance and allow for bungs. Ovalheads are a compromise that providegood clearance, a more attractive

Through-bolting is ideal for maximum security and load bearing. But careful exami-nation of these washers reveals distortion because they’re too thin. In this type ofapplication, choose backing plates for better load bearing and load spreading.

Left—An example of the wrong head. The fastener’s inverted cone head acted like a simple wedge, damaging the fuel tankcomponent. Fastener heads must be chosen as carefully as threads and dimensions. Right—Setscrews are common inmarine hardware applications, and are available in a variety of types and styles. The setscrew on the right is well suited for apropeller coupling, thanks to its seizing-wire orifice. The one on the left is better for the stacked application often found onface-seal stuffing boxes.

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finish, and deeper drive recess, whichresists stripping. (In fastener industryargot, the appropriate term for suchhead failure is cam-out, with strip-ping traditionally reserved for threadfailure.)

Fillister and cheese heads are usu-ally found on small machine applica-tions such as fuel-injection and liftpumps, where the fastener is driveninto a machined cylinder or recess, orwhere greater drive torque is requiredfor limited head-width clearance.

Hex heads have already been dis-cussed above. They’re commonthroughout most vessels, securingautopilot rams and brackets, steeringquadrants, motor mounts, alternators,mast and rigging hardware, and allmanner of equipment and systems.

Square heads are not quite as com-mon, but at least one is often foundaboard inboard-powered vessels:many propeller shafts are secured totheir couplings by one or two hard-ened square-head set screws. Thesesquare-headed screws are speciallydesigned for precisely this use andshould never be substituted with ordi-

nary hex head fasteners, even thoughthe threads are interchangeable.

A few additional points regardingset-screws: Many set-screw heads arepredrilled to accept seizing wire—aworthy addition where loss of thescrew could be catastrophic. I saypredrilled because this hole cannoteasily be drilled in the field after man-ufacture when the screw has beenhardened. If a set-screw lacks a seizing-wire hole, replace it with one that isso equipped.

Finally, set-screws are available in avariety of tip styles: plain, flat, andoval; dogged, cone, and cup. Set-screw tips that are employed on hardsurfaces, such as propeller shafts, areusually cup tipped and the shaft dim-pled to accept this shape; soft materi-als such as aluminum and brass oftenuse cone points. Regardless of style,it’s important that a set-screw tipmatch its corresponding recess.

DrivesA fastener’s drive type is the physi-

cal configuration or pattern on thehead that allows torque to rotate the

bolt or screw into a nut, threadedhole, or pilot hole. Nearly everybuilder, mechanic, and electrician isfamiliar with the most common drives:the blade or common screwdriver,and the Phillips head, both of whichare found on self-tapping, wood, andmachine screws. Today’s professionalmay, however, be faced with moreesoteric drives including the socketrecess drive, also know as Allen; Reedand Prince, a.k.a. Frearson ; squarerecess ; and Torx, to name a few.There are few mechanical blundersless forgivable and more easily avoid-able than a professional choosing thewrong tool to install or remove a fas-tener.

Six-point socket recess, sockethead, internal wrenching, cap head,Allen—all different names for a singledrive style—are common in large andsmall equipment applications such aswatermakers, bow thrusters, andsome electronic gear. They are essen-tially an inside-out hex-head bolt thatutilizes either an imperial or metricmale six-point drive. Allen drives per-form best where a hex head is

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impractical: for example, in limited-clearance applications such as motor-mount brackets, or recessed wells.Because they are especially strongand resistant to cam-out, such drivetypes are often found on high-torqueand critical applications.

Frearson drives are commonly mis-taken for Phillips heads. At a glance,they appear to be similar, but the

Frearson forms a slightly larger X thanthe Phillips, and its recess is notice-ably deeper. This drive is a favorite ofship’s carpenters and joiners becauseof its resistance to cam-out whendriven with a drill. However, it’s alsomore susceptible to stripping andbreakage if over-torqued.

Should you attempt to drive aFrearson head with an ordinary

Phillips screwdriver, it may not beimmediately apparent that they areincompatible; but because the Phillipsdrive is essentially smaller and shal-lower, it often cams out of a Frearson,even under light torque.

Recessed square drives, inventedby Canadian screw salesman PeterRobertson in 1907, were onceextremely popular in wooden boat

Torx, or star (left), and Allen, or recessed hex (right), drives are gaining popularity in the marine industry. These cam-out-resistantfasteners work well where there’s insufficient clearance for a conventional socket drive.

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building and are still among the mostpopular drives in Canada. They’reespecially resistant to cam-out, andthus favored by boatyard carpentersthe world over. Square drives areeven more resistant to cam-out thanFrearsons, and are therefore easy todamage or break if over-torqued.

In 1936, Henry Phillips introducedthe Phillips drive to compete with theRobertson, or square, drive. One fea-ture of the former was that thePhillips driver, unlike the squaredrive, cammed out only when suffi-cient torque was attained—a valuethat lent itself to less-skilled workers.

Top—Looks like a Phillips, but it isn’t.While Frearson-head fasteners are oftenmistaken for Phillips heads, the X isdeeper and wider than on the Phillips,reducing the likelihood of cam-out. Thedrivers are not interchangeable.Bottom—Security Torx and matchingdrive are a variation of the Torx. Thesefasteners are often installed as tamper-proof devices on electronics and othernon-user-serviceable equipment.

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The Phillips drive was quicklyadopted by Detroit automakers,which proclaimed its self-aligningattribute a considerable time-saverover slotted fasteners.

Torx drives are essentially arecessed six-pointed star pattern, thedriver being the male and the fastenerthe female. (The inverse setup isavailable, but it’s far less common.)

Equipment manufacturers ofteninstall this type of drive to deliberatelydiscourage service by inexperiencedor untrained personnel. Thus, elec-tronics and computer equipment and,once again, fuel injection pumps, arefrequently fitted with Torx drives. Aminor variation is the security-styleTorx; it has a small raised section inthe center of the fastener’s headrecess, which fits into a mating recessin the driving tool.

Reading Bolt HeadsThe markings on hex bolt heads

provide critical information about afastener’s strength, chemical composi-tion, and manufacturer. For carbon-steel hex head bolts, the most

common grades are SAE 2, 5, and 8,identified by zero, three, and six hashmarks on the head, respectively. Theirtensile strength ratings are 74, 120,and 150 thousand pounds per squareinch, respectively.

For metric mild-steel fasteners, thecorresponding most commonly foundgrades are 8.8, 10.9, and 12.9, whichare more logically indicated by thesevery figures stamped on the corre-sponding bolt’s head. Their tensilestrength ratings are 8,158 kg/cm2,10,600 kg/cm2, and 12,447 kg/cm2,respectively. This partial list of imper-ial and metric bolt ratings representsthe most common fasteners foundaboard the average recreational, andmany a commercial/military, smallvessel.

For stainless fasteners, bolt-headmarking is more rare than common.Their tensile-strength classes (for stain-less fasteners it’s class rather thangrade) are 50, 70, and 80, representing70, 100, and 118 thousand pounds persquare inch, respectively. Stainless boltheads are rarely marked to indicateclasses. When they are, though, the

class is clearly indicated with thenumbers 50, 70, or 80.

Lest the reader be left with theimpression that metric fastener-headmarkings are entirely too logical,here’s one wrinkle: When marked,302- and 304-stainless metric fastenersare denoted as “A2”; 316-stainless fas-teners are “A4”.

One final head-marking worthmentioning is F593C. Now virtuallyuniversal in the marine stainless fas-tener industry, this designationdenotes stainless hex bolts or capscrews that meet specific parametersset forth by the American Society forTesting and Materials, or ASTM.Compared to ordinary stainless fasten-ers, F593 (F594 is the equivalent clas-sification for stainless nuts) fastenersmeet tensile strength requirementsroughly 20% higher than 18-8 alloystainless, as well as minimum andmaximum hardness requirements.Additionally, F593 guidelines call forspecific alloy requirements, eliminat-ing many common mixtures of 300 or18-8 stainless. The C in F593C denotesa size range from 1⁄4" to 5⁄8", while the

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D suffix is used for 3⁄4" - to 11⁄ 2" -diameterbolts.

Although fasteners sold as 18-8 or304 stainless alloy meet, for the mostpart, self-regulated standards, the

ASTM F593C designation gives engi-neers, builders, repairers, and con-sumers the assurance of an industry-governed specification that falls underthe federal Fastener Quality Act of

1990. The act governs specificationsfor critical fasteners.

Fasteners in the recreational marineindustry are not required to be subjectto the Fastener Quality Act. But, the

Different head markings are often found on stainless steel bolts. The F593C designation is among the most important. THE,common on the heads of many stainless bolts in North America and Asia, are the initials of the manufacturer.

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F593 designation assures builders thata fastener is of consistent, high quality.Note that widespread counterfeiting offasteners has been reduced by thisAct, but there have been instances offasteners marked as F593 whose prop-erties did not meet ASTM specs.

One final caution: hash marks onthe heads of stainless hex fastenersshould not be mistaken for tensilestrength grade indicators. Hash marksindicate tensile strength only on car-bon steel fasteners. Many stainlessbolts were marked at one time withtwo hash marks to indicate they were

Top—A 304-stainless class-70 metricfastener. Stainless fasteners are typi-cally not as strong as their carbonsteel counterparts, so never substitutea stainless fastener for a carbon steelone, particularly on engineered machin-ery components such as engines anddrivetrains. Bottom—A die-cut thread.While many machinists and tool-and-diemakers may argue the point, it’s gener-ally accepted that cut-thread fastenersare not as strong as cold-rolled ones.

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manufactured from 18-8 stainlessalloy.

Machine Screw SizeKnowing how to properly read the

size of a machine screw is a vital skillfor any mechanic, boatbuilder, or yardsupervisor. It’s easy to master onceyou know the basics. The first num-ber in an SAE fastener measurement isthe thread diameter—diameter acrossthe fastener from thread peak topeak—in inches, or fraction thereof.Thread count per inch of threadedlength is next, followed by the nomi-nal length of the bolt (the overalllength excluding the head). Thus, a5⁄16-24 x 1.5 HHCS is a hex head capscrew that measures 5⁄16" in diameter,has 24 threads per inch, and is 11⁄2"long.

Metric fasteners adhere to similarmeasurement protocols, substitutingmetric measurements for imperial.The diameter is usually preceded byan M, such as M8, which signifies abolt whose diameter (measured fromthread peak to thread peak across thefastener’s axis) is 8mm. Finally, the

nominal length is indicated in mil-limeters. Therefore, an M8-1.25 x 25RHMS would be a round headmachine screw whose diameter is8mm, with a thread pitch of 1.25mm,and a length of 25mm. (The termpitch in metric thread measurementreplaces imperial’s threads-per-inchapproach. Pitch is the distance in mmfrom any one point on a thread to acorresponding point on the nextthread when measured parallel to itsaxis. Therefore, a metric screw threadtravels this distance in one revolution.For example, if one turn advances thefastener 1.5mm, then it is called“M1.5.” It’s the distance from onepeak of the thread to the next one.This number is referred to as thepitch.)

Thread-pitch gauges are readily(and inexpensively) available for mea-suring imperial and metric pitches.This, along with an accurate ruler orcaliper graduated in both inches andmillimeters, will allow you to identifyany machine screw or bolt. That, cou-pled with your ability to read hexhead markings, will leave few fasten-

ers indecipherable or unidentifiable.There may still be a few mysteries,

one being bolts devoid of head mark-ings. Metric, imperial, stainless, or car-bon steel bolts whose heads aresmooth and unmarked are SAE grades0, 1, and 2. The specific properties ofsuch fasteners cannot be ascertainedother than from their packaging, orfrom suitable documentation at pointof purchase.

I follow the safe and sensible rulethat fasteners whose lineage cannotbe clearly established should not beused. If you choose to do so, youincur the risk and consequences ofthe fastener’s failure.

Know your fasteners, and chooseyour threads and heads wisely.

About the Author: A newly namedcontributing editor of this magazineand former full-service yard manager,the author now works with boatbuilders and owners and others in theindustry as “Steve D’Antonio MarineConsulting Inc.” His book on marinesystems will be published by McGrawHill this year.

PBB

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