Auto Service World
Feature   June 1, 2000   by CARS Magazine

Fastener strength with repeated tightening

Torque-to-yield fasteners, typically bolts, have become a popular way of holding cylinder heads and some engine accessories to the block. Torque-to-yield fasteners work by actually exceeding the faste...


Torque-to-yield fasteners, typically bolts, have become a popular way of holding cylinder heads and some engine accessories to the block. Torque-to-yield fasteners work by actually exceeding the fastener metal’s yield point, allowing the metal to deform by stretching along its length. Unloading the fastener doesn’t remove the stretch, so torque-to-yields are generally not reused. But what about “regular” non-torque-to-yield fasteners? To an extent, the same rules apply. Conventional fasteners, even when properly torqued, can lose 20 percent of their tensile strength after the first tightening. A second cycle can result in a fastener with up to one third less tensile strength. And when “miked”, the fastener will often show no elongation, since the torque levels were below the metal’s elastic limit. For critical fasteners which are suspected of multiple torque cycles, fastener replacement is the only safe, sure procedure.


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