The wild, weird sci­ence of au­to­mo­tive fas­ten­ers and how to se­lect and use them

Ever been strolling through a hard­ware store or per­haps an auto parts estab­lish­ment and won­dered about the mean­ing be­hind the mark­ings on the hun­dreds of thou­sands of bolt heads on the shelf? Whether you’re cus­tom building, restor­ing an old clas­sic from scratch, or per­form­ing a con­course restora­tion, it is im­por­tant to un­der­stand de­ci­sions cru­cial to your safety out there on the road. One of the great­est safety and aes­thet­ics de­ci­sions is proper fas­tener se­lec­tion. You want the right bolt or screw for the job and you want to be able to trust the source.

Au­to­mo­tive Rac­ing Products (ARP) is here to help you un­der­stand the very pur­pose and func­tion of fas­ten­ers. In or­der to make the right decision when you’re or­der­ing fas­ten­ers, you have to un­der­stand met­al­lurgy and how fas­ten­ers func­tion. Fas­ten­ers pro­vide a threaded clamp­ing func­tion that draws and holds com­po­nents to­gether. With that clamp­ing power comes fas­tener ten­sion and stretch. There’s al­ways a cer­tain amount of “give” in parts and fas­ten­ers when the torque is ap­plied. Where bolts and screws be­come more com­plex is when we get into the ma­te­ri­als they are made of, thread de­sign, and how strength is con­firmed.

ARP ex­plains that fas­tener met­al­lurgy be­gins with grain size in the ma­te­rial. Steel “freezes” from its liq­uid state dur­ing melt­ing from many ori­gins (called al­lotropic) and each one of these ori­gins grows un­til it con­tacts an­other dur­ing the freez­ing (cool down) process. Each of these seg­ments is a grain. In iron and alu­minum cast­ings, these grains are fairly large. When grains be­come tighter (smaller) we get steel. Grains can be made tighter; there­fore, even more of these grains can oc­cupy the same space. This hap­pens by first cold work­ing the ma­te­rial and then re­crys­tal­liz­ing at very high tem­per­a­tures.

Al­loys are spe­cial mixes of el­e­ments that be­come a stronger form of iron, known as steel. Al­loys, like chro­moly steel, do not have to be cold-worked.

What makes ARP bolts and studs op­ti­mum for ex­treme duty is

very fine grain met­al­lurgy, usu­ally ASTM 8 or finer, with 10 be­ing the finest. ARP gives us insight into bolt strength with even more de­tailed in­for­ma­tion. With steel, as the strength goes up, we lose some­thing known as tough­ness. When steel be­comes stronger, it tends to be­come brit­tle and more prone to break­age. Bolt, stud, and nut threads con­trib­ute to brit­tle­ness, ARP tells us.

Now About Bolts

The most com­mon bolt is a “hex” head with six con­tact sur­faces that pen­e­trate a nut or threaded re­ceiver. Fas­tener grade (U.S. or Met­ric) is a bolt’s makeup phys­i­cally and me­chan­i­cally. There are five ba­sic bolt hard­nesses, also known as ASTM rat­ings: Grade 1 (very soft), Grade 2 (soft), Grade 5 (hard), Grade 7 (harder), and Grade 8 (very hard).

A U.S. Grade 1 or 2 fas­tener—your ba­sic hard­ware store peanut but­ter bolt—should never be used on a mo­tor ve­hi­cle in the in­ter­est

of safety. U.S. Grade 1 and 2 fas­ten­ers are low to medium car­bon steel and are gen­er­ally low qual­ity. They break eas­ily. Though the stan­dard is 74,000-psi min­i­mum ten­sile strength, these are the bolt grades to avoid. Light-duty fas­ten­ers, such as body bolts and in­te­rior fas­ten­ers, are gen­er­ally U.S. Grade 5, some­times less. U.S. Grade 5 bolts are quenched and tem­pered medium car­bon steel. Min­i­mum ten­sile strength for these is gen­er­ally 120,000 psi, mean­ing min­i­mum ten­sion load is 120,000 psi.

En­gine, driv­e­line, and chas­sis com­po­nents should be fit­ted with

U.S. Grade 8 fas­ten­ers with­out ex­cep­tion. U.S. Grade 8 fas­ten­ers are high-strength quenched and tem­pered medium car­bon al­loy steel. They’re about as strong as it gets. They can gen­er­ally take 150,000-psi min­i­mum ten­sile strength.

Stain­less bolts vary a lot in ma­te­ri­als and con­struc­tion. Stain­less is a low car­bon steel fas­tener that of­fers ex­cel­lent cor­ro­sion re­sis­tance. Some be­lieve stain­less steel bolts are stronger than Grades 5 and 8 but this isn’t true. Be­cause stain­less is a low car­bon steel it can­not al­ways be heat treated for strength. Stain­less steel bolts are gen­er­ally harder than Grade 2 but not as hard as Grade 5.

Good house­keep­ing is every­thing to fas­tener in­stal­la­tion.

If you take a rusty or dirty bolt and ex­pect easy in­stal­la­tion, for­get it. Threads should be clean when you’re screw­ing things to­gether. At the very least clean up the threads with a wire wheel, re­mem­ber­ing to use eye and face protection. Give hard­ware a run-through with a lu­bri­cant be­fore get­ting down to the fi­nal in­stal­la­tion. And re­mem­ber, dirty or rusty threads vir­tu­ally guar­an­tee dam­aged threads.

Bolt/Nut Dy­nam­ics

While you’re shop­ping for bolts, be think­ing about nuts. A Grade 1 or 2 peanut but­ter bolt is en­gi­neered to work with a cor­re­spond­ing low or no-grade nut. A Grade 5 bolt works with a Grade F nut while a Grade 8 or higher bolt is de­signed for a Grade G nut. Bolt prop­er­ties in­clude al­loy steel, which is heat-treated and typ­i­cally sports a dull, black fin­ish. With steel al­loy you get strength, how­ever, you also get brit­tle. Brit­tle can break. You can ex­pect to see stan­dard hex nuts, jam nuts, flange nuts, star washer, or K-lock-Kep, a lock­nut known as a pre­vail­ing torque lock nut, slot­ted or cas­tle, and ny­lon or Ny­lock nuts. Some of these you’re not likely to find in an OEM ap­pli­ca­tion.

Torquing Fas­ten­ers

When you’re tight­en­ing bolts, it’s vi­tal to lu­bri­cate the threads and take it slow do­ing so. Bolts should al­ways be seated, then slowly torqued in one-third val­ues. Once you have ac­com­plished all three torque val­ues in proper or­der, torque must be checked again. Never jerk a torque wrench to hit break­away torque (click!). Slowly pull into the value de­sired and be pa­tient, wait for the click. When you’re fin­ished al­ways zero the torque wrench if you’re us­ing a break­away.

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