TDC

WHY SHORT FAS­TEN­ERS COME LOOSE MORE EAS­ILY

Cycle World - - News - By Kevin Cameron

When Har­ley-david­son brought out the new de­sign known as the “Evo­lu­tion” en­gine, a ma­jor goal was to elim­i­nate oil leaks and gas­ket fail­ures that had made the pre­vi­ous model the butt of light re­marks. Ear­lier en­gines em­ployed cylin­ders bolted to the crank­case by short bolts through a base flange. Heads were then bolted to the tops of the cylin­ders.

A ma­jor part of the “fix” was to re­place the short base and head bolts with long through-studs that held down both cylin­ders and heads.

Why would this change keep en­gines leak-free? Think of bolts and studs as springs. When you tighten a bolt or the nut on a stud, you are stretch­ing that bolt or stud, and the re­sult­ing ten­sion keeps parts in place and gas­kets tight. Be­cause there are lim­its to how much you can stretch metal be­fore it yields (the usual mis­take here is “I just tight­ened it un­til it started to get loose…”), you can put more stretch into a long fas­tener than you can into a short one. A long fas­tener can better tol­er­ate the heat expansion of the parts they are clamp­ing, whereas a short fas­tener can more eas­ily be per­ma­nently stretched so that it loses part of its clamp load.

Other ef­fects con­tribut­ing to this dif­fer­ence are set­tling of gas­kets (which is why re-torque­ing of head or cylin­der fas­ten­ers af­ter ini­tial en­gine op­er­a­tion is some­times called for) and loss of metal from the slight rel­a­tive move­ments be­tween fas­tener and part caused by vi­bra­tion. A good ex­am­ple of this is the loos­en­ing of the large, thin nuts of­ten used to re­tain en­gine out­put sprock­ets on their shaft splines or of the sev­eral bolts used to re­tain rear wheel sprock­ets.

De­spite the usual pres­ence of a tor­sional shock ab­sorber, built into the clutch bas­ket, the drive from en­gine to rear wheel is not smooth. All the tiny mo­tions that re­sult cause sur­face scrub­bing, each such move­ment mak­ing a zil­lion mi­cro-welds that are bro­ken by the next move­ment. In steel-to-steel-con­tacts of this kind you may find a red­dish dis­col­oration or even red pow­der-the iron ox­ide that re­sults from “fret­tage,” the slight vi­bra­tory weld-and-break action be­tween sur­faces.

Thin sprocket nuts are so no­to­ri­ous for com­ing loose that many man­u­fac­tur­ers pro­vide tab wash­ers to pre­vent a grad­u­ally loos­en­ing nut from com­ing off en­tirely. Oth­ers avoid this by slid­ing the sprocket onto its splined shaft and retaining it with a cir­clip— no nut to loosen.

I learned long ago that when the rear sprocket on the bike af­ter last week’s event is the same one needed to­day, I must still go through the process of un­bend­ing the tab wash­ers or safety wire retaining the sprocket bolts then re-torque­ing and se­cur­ing with tab wash­ers or wire. Why? Be­cause I’d found that a week­end of rac­ing mileage had loos­ened all the bolts. A look at where the sprocket pressed against the wheel showed ev­i­dence of back-and-forth mo­tion that had grad­u­ally re­sulted in loss of bolt ten­sion.

At the Cana­dian GP in 1967 I saw the late Ar­turo Magni make a kind of “rite” out of chang­ing the rear sprocket on Agos­tini’s MV 500 Triple. There were no bolts! Instead, the sprocket fit­ted onto sev­eral short drive pins in the wheel hub and was held in place by a sin­gle large cir­clip (which was sub­se­quently wired in place).

Then there was the case of Kawasaki’s F5 off-road 350 sin­gle, which was for a time le­gal in AMA’S 250 GP class. Driving it­self for­ward as it did by a se­ries of large com­bus­tion thumps, there was trou­ble keep­ing the pri­mary pin­ion on the crank­shaft. This was the re­sult of a cou­ple of fac­tors. Pri­mary was the thump­ing, which in­evitably produced rel­a­tive mo­tion be­tween pin­ion and shaft, and sec­ondary was the short­ness of the

fas­tener—re­sult­ing in too lit­tle “stretch” to main­tain tight­ness. A first-try tab washer just sheared off; the vi­bra­tory “un­screw­ing torque” was too much for the thin metal. The next try: a thicker washer.

The re­sponse of vet­eran racer/ builder (and 1969 250 GP World Cham­pion) Kel Carruthers was, “It could un­screw once for any of us, and we might not get the fix right the first time, but by God I’d weld the thing to the shaft be­fore I’d let it come loose again!”

Per­haps that’s why Na­gato Sato, who de­signed Kawasaki’s next 250 class en­try—the tan­dem-cylin­der Kr250—gave it pressed-on crank phas­ing gears. No vul­ner­a­ble keys, splines, or tab wash­ers giv­ing up at 12,000 rpm for him.

Po­laris/in­dian may have had to deal with some­thing sim­i­lar in se­cur­ing the ex­ter­nal fly­wheel (made in three weights to tai­lor rear-tire hookup) of its FTR750 race-only dirt-track en­gine. The slight­est back­lash in a splined con­nec­tion en­cour­ages fret­tage and loss of fas­tener torque.

Then why did the last gen­er­a­tion of two-stroke mo­tocross en­gines at­tach their cylin­ders and heads not with ad­vis­able long elas­tic through-studs (as found on all 1970s TZ road­race en­gines) but with short base bolts, with the head then bolted to the cylin­der? In this case, de­sign­ers had pre­vi­ously found those long stud tun­nels right in the way of where they needed to route next year’s trans­fer ports or sub-ex­hausts. In some cases tuners had re­sorted to first mod­i­fy­ing the ports then press­ing thin steel sleeves into the stud tun­nels to seal the places where they’d cut into them. Short fas­ten­ers—de­spite their propen­sity for com­ing loose—could more eas­ily avoid in­ter­fer­ence with ports.

In many other cases when short fas­ten­ers pos­i­tively must not come

AT THE CANA­DIAN GP IN 1967 I SAW THE LATE AR­TURO MAGNI MAKE A KIND OF “RITE” OUT OF CHANG­ING THE REAR SPROCKET ON AGOS­TINI’S MV 500 TRIPLE.

loose, lock­ing de­vices such as Pal­nuts, safety wire (where there’s room to use it!), or red Loc­tite must be used. Each of the 18 46-pound air­cooled cylin­ders on a WWII P-47’s R-2800 ra­dial pis­ton en­gine was held to the forged alu­minum crank­case by 20 short studs and nuts, each se­cured by a Pal­nut (a type of lock­nut formed from spring steel). No won­der aero­space has pretty much aban­doned safety wire in fa­vor of self-lock­ing fas­ten­ers; imag­ine reach­ing 20 inches into that for­est of cylin­ders to wire all 360 nuts.

The first time I tried to se­cure a slot­ted cam sprocket to its cam I fool­ishly ap­plied the oth­er­wise usual drop of red Loc­tite. It came loose be­cause 1) the sprocket bolts are very short and 2) cam drive is a very “lumpy load.” Point taken: I should have drowned those bolts in red. No short­cuts! Build to be sure.

Learn and use best prac­tice, and up­date it with your own hard-won ex­pe­ri­ence.

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