Mean­ing: Read the In­struc­tions!

Drag Racer - - Contents - Text by Bill Hol­land Pho­tos cour­tesy of ARP

BEEN THERE, DONE THAT. How of­ten have you heard that ex­pres­sion? Fact is, the pop­u­lar say­ing can be ap­plied to many things re­lated to as­sem­bling rac­ing en­gines, but there should also be a big red cau­tion flag waved be­cause in­dus­try­wide im­prove­ment in met­al­lurgy, ma­chin­ing ac­cu­racy and tech­nol­ogy have changed things. A process once held as gospel might be ob­so­lete to­day.

The grav­ity of this sit­u­a­tion was re­cently pointed out by the folks at fas­tener man­u­fac­turer ARP, who have made a con­certed ef­fort to call at­ten­tion to the changes and en­cour­age racers to make sure that they ad­here to the most cur­rent en­gine-as­sem­bly prac­tices put forth by man­u­fac­tur­ers. To that ex­tent, ARP has up­dated and ex­panded its in­struc­tion sheets and has posted them on­line for easy ac­cess.

A case in point in­volves in­stalling head studs. To­day’s cylin­der heads are ma­chined to much closer tol­er­ances than in the past. The sur­face fin­ish of the mount­ing pads is now very smooth, as op­posed to the rel­a­tively rough ma­chin­ing of years ago. Ac­cord­ingly, if you get any lu­bri­cant un­der the flat washer, it can turn it into a bear­ing of sorts and greatly af­fect torque­ing ac­cu­racy.

Re­gard­ing sur­face fin­ishes, there are enough dif­fer­ences be­tween a pol­ished stain­less-steel fas­tener and a black­ox­ide-coated chrome-moly bolt or stud that in­struc­tion sheets may re­flect vary­ing preload spec­i­fi­ca­tions.

Thanks to de­vel­op­ments in mea­sur­ing ac­tual fas­tener preload­ing, more at­ten­tion has been fo­cused on the phe­nom­e­non known as “preload scat­ter.” When a fas­tener is torqued, only a per­cent­age of the en­ergy is used to stretch the bolt or stud, which is where the clamp­ing force comes from. Think of a fas­tener as a spring, with the re­bound, when stretched, pro­vid­ing the preload. Much of the en­ergy is ac­tu­ally usurped over­com­ing fric­tion, which can largely be at­trib­uted to the thread qual­ity and clean­li­ness, as well as the lu­bri­cant that’s em­ployed.

ARP’s en­gi­neer­ing team spent years care­fully an­a­lyz­ing preload scat­ter us­ing so­phis­ti­cated com­puter-con­trolled mea­sur­ing equip­ment and dis­cov­ered huge dis­par­i­ties from one lu­bri­cant to an­other—up to 30% or more. This lead to the de­vel­op­ment of ARP Ul­tra-Torque Fas­tener Lu­bri­cant, which is so con­sis­tent, it de­liv­ers 95-100% of the de­sired preload on the first (and any sub­se­quent) pull of the torque wrench.

Prior to this, the only way to en­sure preload ac­cu­racy was to torque the fas­tener to the de­sired set­ting, back it off and retighten. Be­tween three and six cy­cles were usu­ally re­quired to mit­i­gate the fric­tion be­tween the threads, bear­ing sur­face and lu­bri­cant to en­sure proper preload­ing.

Hav­ing the proper preload is im­por­tant for a cou­ple of rea­sons. Not hav­ing suf­fi­cient clamp­ing force on a head gas­ket, for ex­am­ple, ex­ac­er­bates its fail­ure; and hav­ing vary­ing clamp­ing loads on ad­ja­cent studs or bolts can dis­tort a cylin­der’s bore and ac­tu­ally have an ef­fect on pis­ton ring seal­ing.

The rec­om­mended pro­ce­dure for in­stalling a head stud or bolt to­day is to thor­oughly clean the threads us­ing a spe­cial chaser tap on the block that won’t cut threads, fol­lowed by care­fully re­mov­ing any de­bris and prop­erly lu­bri­cat­ing the fas­tener threads, as well as the un­der­side of the nut (or bolt head).

ARP rec­om­mends achiev­ing the de­sired torque read­ing in three in­cre­men­tal steps but also cau­tions that torque wrenches need to be checked for ac­cu­racy. For years ARP has of­fered free torque wrench checks at its dis­play on the Man­u­fac­turer’s Mid­way at all NHRA Na­tional events and dis­cov­ered huge dis­par­i­ties in ac­cu­racy. A com­mon prob­lem, ac­cord­ing to ARP’s tech reps, is em­ploy­ing a torque wrench as a breaker bar. Us­ing it to re­move a fas­tener is a

big no-no. Torque wrenches are del­i­cate in­stru­ments and should be treated ac­cord­ingly and cal­i­brated pe­ri­od­i­cally.

When torque­ing the head stud or bolt, make sure the washer is not ro­tat­ing. Wash­ers with pat­terns ma­chined into them must be in­stalled with the pat­tern fac­ing the block.

An­other po­ten­tial prob­lem area con­cerns up­grad­ing the qual­ity of a fas­tener and re­ly­ing on OEM torque specs. Of­ten­times, fas­ten­ers with in­creased ten­sile strengths re­quire ad­di­tional preload­ing to achieve the de­sired clamp­ing force. Rely on the data for that spe­cific fas­tener.

In a re­lated mat­ter, when clamp­ing force is in­creased, it’s vi­tal to un­der­stand ma­te­rial will be af­fected. For ex­am­ple, hav­ing higher strength main bolts or studs that re­quire in­creased preload­ing can af­fect the shape of the cap and pinch the bear­ing. It’s im­por­tant to align, bore and hone a block with a fas­tener up­grade. The same type of thing hap­pens with con­nect­ing rods, so higher preload­ing war­rants re­siz­ing the rod.

There are many other in­stances where old prac­tices can be prob­lem­atic. Take head gas­kets. For years, us­ing one par­tic­u­lar spray/sealer was the hot tip. Now some man­u­fac­tur­ers of MLS (multi-layer steel) gas­kets es­chew the prac­tice.

The bot­tom line is you should rely on the in­struc­tions and not do some­thing be­cause that’s the way it’s al­ways been done.

As a part­ing thought, for many years, rac­ing icon Smokey Yu­nick was a val­ued con­sul­tant to ARP and rep­re­sented the com­pany at trade shows. In his book, Power Se­crets, Smokey notes, “I don’t care what kind of en­gine you’re build­ing, be­fore you even con­sider ‘fi­nal as­sem­bly’ you should do a com­plete mock-up as­sem­bly of the en­gine and all the en­gine com­po­nents.” Smokey went on to ad­vo­cate care­fully check­ing all clear­ances, as bolt head height and the nut/ washer com­bi­na­tion can af­fect how things fit over­all.

Af­ter all, it’s the lit­tle things that count, and dili­gence can of­ten spell the dif­fer­ence be­tween a project that goes smoothly or winds up with ma­jor prob­lems.

… it’s the lit­tle things that count, and dili­gence can of­ten spell the dif­fer­ence be­tween a project that goes smoothly or winds up with ma­jor prob­lems.

Use a spe­cial chaser tap to clean threaded holes.

The cylin­der head stud/bolt mount­ing pad on a pop­u­lar af­ter­mar­ket SBC cylin­der head, and the other pad on a new Coy­ote en­gine. You can see how the newer heads are fin­ished bet­ter and pro­vide lit­tle grip for the washer.

Check your torque wrench for ac­cu­racy.

Use lu­bri­cant on the un­der­side of the bolt head (or nut), not on the un­der­side of the washer and on fas­tener threads.

Fol­low man­u­fac­turer spec­i­fi­ca­tions when ap­ply­ing preload torque. Use a stretch gauge on con­nect­ing rods.

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