How Fasteners Ought to Work

[Hugh Janus]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Threaded fasteners are actually very stiff springs that hold assemblies of parts together by being stretched in the act of tightening. The stretching I’m talking about here is elastic stretch: When the fastener is unscrewed, the stretch put into it by tightening disappears and the part returns to its original dimensions.

The amount of stretch can be visualized by considering the classic recommended method for tightening the connecting-rod bolts of a 500cc Triumph Twin. A micrometer was required for this, of a capacity great enough to measure bolt length. Measure bolt lengths before installation and write down the numbers. Add 0.004 of an inch to those lengths and, once the rods, their bearing shells, and big-end caps are installed on the crankpins, tighten the two bolts in stages until they are stretched to those lengths.

The various other methods of fastener installation are all trying to accomplish this same goal: to stretch the fastener by a controlled amount. The usual method is to assume that installation torque (so many pound-feet or kilogram-meters of it) will translate reliably into controlled fastener stretch. But will it? Are the parts clean, or is there some corrosion or dirt on the threads? Are the threads dry or lubricated? Because the method described in the service manual is approved by the manufacturer, we assume it comes close enough to being accurate. But the goal of tightening is fastener stretch—which is exactly like spring preload in a motorcycle’s suspension.

The amount of stretch (the 0.004 inch in the case of 1960s Triumph rod bolts) is of course proportional to the length of the fastener, because what we are doing in tightening is trying to load the material in the fastener to a desired stress level. Too little tightening and we are wasting the strength of the fastener. Too much and either we may permanently stretch the fastener (causing it to yield in plastic deformation) or risk that a high-strength fastener, so tightened, will crush the softer material of an aluminum or magnesium part it is fastening.

One role of the amount of stretch is to hold parts together against the stresses they bear in service. Another is explained by the oil leakage problems Harley-Davidson was having before the original Evo engine was introduced in 1984. Cylinders were previously attached to the crankcase by a bolting flange at the base, retained by a number of short fasteners. Because engine vibration exists, the inertia of cylinders and heads as crankcase shaking yanked them this way and that caused tiny scrubbing motions of cylinders, base gaskets, and crankcases, resulting in some loss of bolt stretch. Because such short bolts could not be safely stretched very far, it wasn’t long before gasket scrubbing from vibration (making the gasket thinner) rendered some base bolts quite loose, and oil leakage—by 1980 no longer considered charming and unavoidable—began.

This, it was explained to me by a member of the Evo design team, was why they switched to fastening both the cylinders and heads by means of long studs that passed through both to be nutted up at head level. By greatly increasing the total amount of stretch in longer fasteners, they would remain adequately tight through a greater loss of preload from gasket scrub. And measures against gasket scrub certainly exist, such as doweling parts together so they cannot fret against each other.

They warned against picking up bare crankcase halves by using the studs as if they were a frying-pan handle—they can be easily bent, which does them no good.

Related: The Long And The Short of Motorcycle Fasteners

The shorter the bolt, the greater the percentage of its length stretches when overtightened compared to a longer fastener turned the same amount.

The very same happened with the big radial piston engines of World War II, which in the 10 years following the war established the worldwide commercial air travel network. To avoid the considerable problem of getting enough bolt pressure to seal head gaskets on giant bores of 5.750 to 6.125 inches, their heads were permanently screwed-and-shrunk onto forged steel barrels which had upset-forged base flanges. On the 18-cylinder Wright R-3350 engine, each base flange was secured to the forged steel crankcase by 21 short bolts. And yes, they did not stay tight, because they were too short to be given enough installation stretch. Yes, there was vibration and movement—aircraft powerplants must be light enough to fly yet strong enough to arrive. That means that all the metal in those engines was worked hard, resulting in cylinders waving about as the side thrust resulting from combustion and con-rod angle hit them.

So the makers took the desperate measure of locking each fastener; on Pratt & Whitney engines this took the form of Pal-Nuts, formed from spring steel and tightened against the cylinder nuts. Because no base gasket could survive, big O-rings more or less sealed the cylinders to the crankcase. Pilots arriving at destination might change into fresh shirts before deplaning, but if so, they were careful never to walk under an engine because of the reliable leakage of black oil that was constant. Some idea of oil consumption may be had from the welded aluminum 100-gallon oil tank mounted behind each of the four engines of a C-124 Globemaster.

Take the fastener torque specs in the service book seriously—the manufacturer knows the product. I remember that a supplier of high-grade driveline pieces was having mystery trouble with a customer whose wheel studs kept breaking. When their engineer visited the man he asked him to demonstrate his tightening procedure. Walking to his toolbox he extracted a respected brand of torque wrench and set to work. He carefully brought the nuts up to torque in three stages. This all looked by-the-book. But once all were at the recommended installation torque, he gave each one an extra quarter turn, permanently stretching all the studs and weakening them. The cause of the failures was that extra quarter turn.

“What are you doing?!” the engineer remonstrated. “You had them all nicely torqued and then this?”

“Uh, well, y’see I don’t want ‘em to come loose.”

Again: Please follow manufacturer’s instructions in tightening fasteners!

Bear in mind that there is another scheme for fastener stretching called “torque to yield” (TTY). On the assembly line, the device torquing such fasteners tightens them until it senses the beginning of yield (torque peaks, then starts to drop). Such fasteners are not normally reusable.

Another temptation is the constant desire to “Do something for your bike.” This sometimes takes the form of buying $32-a-quart special engine oil, or replacing wheel bearings with pumped-up alternatives containing silicon nitride balls instead of penny-a-dozen 52100 steel balls. And one I’ve been guilty of is the fastener upgrade—replacing the hardware-store-quality original fasteners (60,000 psi stuff) with Grade 8 or stronger replacements (higher-spec materials are available all the way to four times that of hardware grade). The temptation when doing this is to torque the stronger fasteners to the higher installation torques of which they are capable. But the manufacturer has already designed the product to be oil-tight and durably assembled using the OE fasteners at their recommended torque. Result of higher torque? Metal under the screw heads is needlessly crushed.

And what about such mechanical restraints as safety wire, tab washers, Loctite, and cotter pins? Rather than hold forth about this, I’ll just suggest that for the few still interested there are industry sources of information.

Occasionally special conditions arise in which service book torque recommendations haven’t worked. One such was the special 10mm bolts that retained the crank drive gears on Yamaha TZ750 race engines. At the book-recommended torque of 35 lb.-ft., they loosened, began to scrub, and the No. 2 and No. 3 cylinders could then drink the oil out of the gearbox. Cheerful, practical Don Vesco told us all that his remedy was to basically double the installation torque. How could that work? Because in this case, it was the thin, large-diameter bolt heads that were deflecting, taking the place of a long bolt or stud shank in providing stretch. They were dishing. And it worked.

Source

[Pedro]

@ lone amigo!!!!!!

help!!!!!!’

[Buckster]

Disappointed to see no mention of torsional deformation.