Thunder Bolts
Thunder bolts
TDC
Kevin Cameron
A CRUCIAL ITFM IN EVERY MODERN motorcycle engine is its connecting-rod bolts. Inertia forces in a running engine increase as the square of rpm. Tooling along a backroad on a warm afternoon at 2000 revs, then revving out each gear to 13,000, involves an increase in revs of six-and-a-half times, but a stress increase in the reciprocating parts of 42 times. To keep the con-rod caps in place under this kind of stress, the rod bolts must withstand being torqued to produce an even higher clamping force.
That is why they are not made of the steel you find in hardware-store bolts, which has a tensile yield strength of around 60,000 psi. Higher-class fasteners, like the ones we buy to customize our bikes with, may display about twice this strength, or in the 100,000to-120,000-psi yield range. Con-rod bolt materials begin at this level.
Picture what happens if one of the two bolts on a con-rod cap in a race engine breaks. Now the cap is secured at one side only, and the next time the piston reaches TDC, it won’t stop. Instead, its inertia will easily bend the con-rod cap and remaining bolt, so the piston keeps going and smashes into the head. This impact destroys the valves, tappets and anything else in the area. The sudden load on the other bolt will bend and break it, and in the next few milliseconds, the crankcase will become a nasty place. The loose cap gets swept around the case by the crank journal and/or counterweights, very likely ending up jammed against the inside of the case. Compared with the kinetic energy of the spinning crank, the bursting strength of the crankcase might as well be that of dry sand. BAM, a hole appears and mangled parts come shooting out, together with a quantity of oil that may complicate the control of the motorcycle in the next few seconds.
The piston and rod, bouncing off the head and having been given a good downward jerk by the one remaining bolt in the instant of its failure, join the happy throng getting processed by the remaining energy in the crankshaft. Presently, the engine stops and there is silence. For want of a bolt, the engine is scrap. Crank, head, cases, cylinder-all junk.
Having considered that unappetizing prospect, you can see why engine builders pay special attention to connecting-rod bolt quality.
The wonderful property of metals is that they bend or stretch before they break; they are ductile. Metals are “glued” together by the gas of free electrons that permeates them. Because of these electrons, the atom-toatom bonds in a metal can break at one point, then reform at another, as strong as before. In non-metals, when a bond breaks, the electrons that mediated the bond go back to their tight little orbits and the bond stays broken. The result is a fast-propagating crack. This is why we don’t make connecting rods or their bolts from materials like granite, which have no ductility-no stretchability-at all.
Sadly, as most steels are heat-treated to higher strengths, you must give up more and more of this valuable ductility. High-carbon steel, heattreated to very high strength, can become brittle almost like glass. Con-rod bolts of such a material might work well so long as loads were smooth and normal, but add an unscheduled minor catastrophe, like a seized and spun bearing. The bearing shells, if starved for an instant of oil, will seize to the journal, then rotate messily in the con-rod instead, applying impulsive loads to the rod bolts. A strong-but-brittle bolt will break, and a small, repairable failure becomes a complete engine wreck.
This sent engineers scurrying to their phones to call their favorite metallurgists; what can you give us that will provide very high strength in the 200.000psi range, but which will stretch rather than snap?
Fortunately, such substances do exist, developed for use in gas turbine engines. As you’d expect, they’re not cheap. These multi-phase metals display very high strengths-but with the ability to stretch considerably before failure. Where a super-hard carbon steel might elongate 5 percent before failing, a multi-phase material displays 30 percent or more elongation. It takes lots of energy to stretch a bolt made of
265.000psi tensile material any distance; that energy acts as a barrier to catastrophic con-rod bolt failure.
These materials begin as a single phase, meaning that only a single type of crystal structure is present within the metal. This phase is ductile, and before final processing, it displays a tensile strength of 100,000 psi or so. Hardness and strength are then developed by cold-working-squeezing, mashing or stretching the material. This causes conversion of some of the original, ductile phase into tiny platelets of a more densely packed, harder phase. The more vigorously the metal is worked, the more of its original ductile phase is converted into the harder phase. The result is a ductile matrix, permeated with tiny hard “bricks.” Under stress, local yielding of metal takes place as atom-toatom bonds break, then reform with new partners. A line of such breaking and reforming of inter-atomic bonds is called a dislocation, and sufficient stress will drive it through the metal. The sum of all these tiny motions is the yielding of the part as a whole. The hard bricks of the second phase in a multi-phase material act as barriers to the movement of dislocations. By preventing yielding, they increase hardness and strength. Yet the presence of the ductile matrix maintains the ability to stretch if the applied force is great enough.
These materials are costly because they contain large percentages of scarce cobalt, available mainly from Zaire, in Africa. However expensive the material, con-rod bolts made from it are regarded as essential by many builders of racing engines. As one fastener engineer remarked to me, the strength and sophistication of these materials may be overkill, but cobalt and nickel can be good insurance agents.
