Threaded Fasteners on Your Motorcycle

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Threaded fasteners can hold things together with great force but can be removed with relative ease. (Cycle World Archives/)

The great thing about threaded fasteners is that they combine the ability to hold things together with great force (such as keeping head gaskets from blowing) with at least the possibility of being easily removed when parts inside a machine must be reached for service.

Knights of olden tyme, eager to avoid neck problems caused by jousting with a long, pronged lance against similarly armed and mounted opponents, fitted a quick-detach metal brace from their helmets to the middles of their backs. This brace was held in place by a large wing nut threaded onto a projecting stud. As soon as the match was over, a squire could spin off the wing nut, allowing Sir Knight to turn his head and gaze upon his lady fair. Five hundred years ago. Those threads were made by hand filing.

A one-piece crankcase would certainly be stronger than the bolted-together variety, but there has to be a way of installing or replacing the crank and other parts. We make complex machines such as engines in many parts, held together with threaded fasteners. Although the concept of the screw is very old, its wide use had to await the industrial revolution of 1712 and onward, and specifically, the screw-cutting lathe of Henry Maudslay, 1797-1800. Without a means to produce threaded parts cheaply and in large numbers, structures and machines were assembled in traditional ways: with nails, wooden pegs, rivets, and knock-out wedges.

A threaded fastener is in fact a very stiff spring that is preloaded in place, elastically stretched, by means of the threads. The resulting tension holds the parts together, and friction between areas of contact (under the heads of bolts or nuts, and between the thread surfaces) keeps the tensioned fastener from unscrewing.

Before machine-made bolts and nuts became available in quantity, ships and other large wooden structures were pegged together or assembled by mortise-and-tenon joints. Just try extracting a lignum vitae peg, expanded by being kept wet, out of its place in a ship’s hull. Try un-hammering a nail, or un-riveting a rivet. But with threaded fasteners, the process is easily reversible.

Fasteners must be kept properly tight to prevent the material in them from being rapidly fatigued by deep stress cycling. The longer a fastener is—such as the studs that retain cylinders and heads on certain engines—the more it is stretched during installation, and the greater the motion the bolted joint can tolerate without loosening. A classic example was Harley-Davidson’s switch from the base-bolted cylinders of their Shovelhead engine of 1966-84 to the late Evo, which employed long through studs. Normal vibration would scrub the base gasket under a “shovel” cylinder. As it was only possible to put a relatively small stretch into the base fasteners, this stretch relaxed, leading to oil leakage. To correct this, the Evo was given long studs, rooted in the crankcase and passing through both cylinder and head. The much greater installation stretch of which such long studs were capable meant the pressure they applied to gaskets did not significantly decrease over the life of the build, preventing gasket damage and leakage.

Similarly, the very large cylinders of air-cooled radial aircraft engines were base-mounted with such short fasteners that anti-loosening Palnuts had to be installed on top of the normal retaining nuts to keep vibration from loosening them. Even so, flight crew in white shirts took care never to walk under those great monsters for fear of the black oil constantly dripping from them.

Often for cylinder head fasteners a tightening sequence is given, usually in steps of increasing torque. The reason for this is to prevent distortion of the parts by unbalanced local fastener pressure. Follow the sequence given—after all, who knows the engine better than its manufacturer?

Although for critical fasteners (such as the connecting-rod cap bolts that fasten each rod to the crankshaft) we usually measure installation torque with a torque wrench, what we really want to achieve is knowledge of _how much the shank of the fastener has been stretched in the process_. Triumph’s method of installing con-rod cap bolts was to measure the length of the bolt with a micrometer before installation, then install the bolts and tighten until they measured 0.004-0.005 inch longer. If the bolts were, say, 2 inches long, then the percentage of stretch would be 0.004 ÷ 2.0 = 0.002. This is the strain to which those bolts were tensioned.

This method obviously cannot be used in situations in which there is no access to both ends of the fastener, so we approximate by using a torque wrench. The torques given in service manuals are usually for dry rather than lubricated threads. Oiling the threads increases the degree of fastener stretch achieved by a given torque, and coating the threads with a dry lubricant such as molybdenum disulfide would increase it even more. Don’t try to “improve” on what the manufacturer knows.

Installation torques given in service manuals are there to be used. Adding more torque “just to be sure” often has the opposite effect, weakening the fastener such that it fails prematurely. The story that comes to mind concerns the wheel studs on a well-regarded make of hot-rod rear axle shaft. A certain customer reported so many failures that a service rep was sent out to interview him. The rep then asked the builder to show him the procedure he was using to torque the fasteners. He produced a name-brand torque wrench and torqued the studs in steps as called for in the maker’s booklet, as the rep looked on. Then, once all were at the recommended torque, he gave each one an extra quarter turn.

“What are you doing?” cried the rep. “You’ve…you’ve just wrecked them all by overtorquing!”

The bewildered builder then mumbled, “Well, see, I didn’t want ‘em to come loose.”

Yet that is precisely the result that his method produced—they broke.

Another of my favorites is the engine builder who tightens every Phillips screw with repeated hard blows on a hammer-driver, causing the soft metal of the case cover being installed to be squeezed out from under the extreme pressure of the fastener to form a kind of little cup. There is no need for such abusive installation!

Yet another is the builder who understandably wishes to avoid the usual cam-out problems with Phillips head fasteners and has replaced them with super-strength socket-head cap screws in Grade 10. He then torques them to the screw manufacturer’s higher recommended torques, giving the same result—the soft metal of the part being fastened is squdged out from under the extra-heavy pressure of the fastener. Unnecessary! Torque the fasteners to the specs given by the engine manufacturer!

The reason big-bore kits for engines often come with larger-than-stock studs is that the extra area of the bigger bore produces greater combustion force, which tends to uproot or stretch the stock studs. Just adding more torque to the existing studs is not a solution; stock installation torque is already taking all they have to give. Back when the late Don Tilley was trying to stop his big racing V-twins from blowing head gaskets at 13-to-1 compression, he just kept making the studs larger in diameter until the warning squeak of a blown gasket wasn’t heard again. I believe his final studs were a full half-inch!

Some critical fasteners are normally replaced at rebuild with fresh ones because the originals have accumulated fatigue damage. The usual candidate is con-rod cap bolts. Consult the service book, not that most variable of information sources: “my buddy.” I have seen enough engines rebuilt and bikes rewired by customers’ “buddies.”

Let the service book be your guide.

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