35 friction from one torque cycle to subsequent cycles. Clamp load scatter generally reduces after multiple cycles because the friction levels out and becomes more consistent. Qualified Products List: A government requirement that simply mandates that bolts be manufactured only by companies which have qualified by making bolts that have been submitted for testing and approval to a government agency. ARP has qualified for this list. Quench & Temper: Amethod of heat-treatingmartensitic steels.Theparts are heated into the austenitic range (usually above 1450˚F) then quenched into water or oil. This leaves the part in a very hard martensitic condition which then must be tempered by heating at lower temperatures (between 350˚F and 1200˚F), depending upon the steel and strength desired. Reciprocating Load: The acceleration force exerted on a connecting rod due to the up and down motion of the piston and its associated mass ie; wrist pin, rings, small end of the rod. Stretch: The increase in length of a bolt when installed with a preload. Stress: The load applied to a part divided by the cross-sectional area of the part, usually expressed in pounds per square inch (psi). Stress Corrosion: This is a special form of hydrogen embrittlement in which the metal is attacked while under stress. Without the stress the crack will not move. But under stress the crack moves and corrosion takes place at the freshly opened crack face. Stress Ratio: The ratio of the minimum stress to the maximum stress in a structure which is subject to fluctuating loads. Stress Riser: You have a notch, ding or some change in section size, so now the stress at these points is increased above nominal stress. Compare this kind of stress to the flow of water in a river. When the river hits a narrow point it flows faster. Perhaps there is a rock in the middle – the river flows faster around the rock. The stress at these points can be so high that the part will fail – even though the average stress on the part never exceeded the tensile strength of the part. S.D.F.: Seam and defect free. A designation for premium steel. This is typically the highest grade available, and is the only steel used by ARP. Thread Engagement: This refers to the number of threads engaged in a nut or threaded hole. Full engagement, meaning all the female threads are engaged, is a desirable configuration to maximize fatigue strength. Ultimate Tensile Strength: The maximum stress that a particular material can support without breaking. It is expressed in terms of lbs. per square inch, and is measured by means of a tensile test. The maximum force (lbs.) that a test specimen can support is divided by the cross-sectional area (square inches) of the specimen, the result is ultimate tensile strength in psi. Torque Angle: A method of tightening a fastener relative to the amount of degrees turned once the underside of the bolt head or nut face contacts the work surface. This procedure is suitable for engine assembly only when the installation has been calibrated in terms of bolt stretch relative to the exact application (the amount of compression of the clamped components is critical). UHL: Means “Under Head Length.” The distance as measured from tip of the fastener to a place directly at the base of the head. Yield Strength: The stress at which a given material or component exhibits a permanent deformation (i.e. “takes a set”). When the load that caused the stress is removed, the part will not return to its original dimensions. If you exceed the yield strength of a fastener, the fastener is ruined and must be replaced. Glossary of Tech Terms FASTENER TECH
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