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Performance & Hotrod Business - April '15

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46 n Performance & Hotrod Business n April 2015 PERFORMANCE connecting rod material itself during bolt clamping. As the bolt is tightened, the head of the bolt will tend to embed itself into the rod, slightly compressing the stock material of the connecting rod under the bolt head. OE production rods are typically softer, allowing the head of the rod bolt to sink into the rod, until the material under the bolt head "work hardens" under compres- sion. ARP notes that the bolt stretch speci- fication is based on the bolt itself, and not on the compression of the rod, since we can't accurately predict what the rod does in this state. Since too many variables exist in terms of rod bolts and connecting rods, we can't draw any generalized conclusions regard- ing ideal connecting rod bolt stretch. However, to use the Chevy small-block 350 as but one isolated example, ARP typi- cally looks for an installed bolt stretch of 0.0063 inches. Since each engine/rod/bolt application differs, we cannot assume that ideal bolt stretch would be the same for any given application. Always refer to the bolt stretch specifications included with the new rod bolts. (Note: OE rod bolts will typically not feature the dimples at the head and tip that allow the use of a stretch gauge. Only high-performance aftermarket rod bolts will provide this feature.) Torque-Plus-Angle I'll touch on this subject briefly. Many late-model OEM engines specify a torque/ angle method for rod bolt tightening. An initial torque value is achieved, (obviously using a properly calibrated torque wrench). This will establish a spe- cific and initial level of clamping force. Final tightening takes place by continu- ing to rotate the bolt head by a specified number of degrees. This method, developed by OE engi- neers, theoretically eliminates the variable of thread friction. Engineering research has determined that continuing to tighten the bolt a certain number of degrees will stretch the bolt into its desired range of elasticity for optimum clamping force. Degree tightening can be accomplished by several different approaches, including When using new OE rod bolts that are specified for torque-plus-angle tightening, torque to the specified value, then continue to tighten by rotating the bolt head the specified number of degrees. This angle gauge connects between the socket and a ratchet. Set the pointer to zero and watch the needle while slowly tight- ening the bolt. The flex clamp shown here is part of the angle gauge shown in this example. This prevents the gauge base from accidentally rotating. A superior type of tool is a digital torque wrench that is also capable of performing angle tightening, such as Snap-on's TechAngle wrench. Combination torque/angle digital tools are now available from several manufacturers. The torque format can be selected (foot-pounds, Nm or inch-pounds). With torque selected, adjust the value desired. Once the selected torque is achieved, the tool will provide an audible and visual indication. If angle tightening is needed, press a button to select the angle mode and adjust for the desired angle. Again, when this angle is obtained, the tool will provide an alert.

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