Performance & Hotrod Business - April '15

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Page 44 of 117

April 2015 n Performance & Hotrod Business n 43 if the published torque value is 70 foot- pounds, torque the bolt to, say, 60 foot- pounds initially. Then, with the stretch gauge still zeroed when the bolt was gauged in its relaxed state, place the gauge onto the bolt to see how far it has stretched. Remove the gauge, apply additional torque and check with the gauge again, repeating this until the recommended amount of stretch has been achieved. The stretch gauge must be zeroed for each indi- vidual bolt while the bolt is in its relaxed state. Even if the builder decides to torque to the full recommended value, it's wise to double-check the installation with a stretch gauge simply to verify that the bolt has been stretched to the specified amount. The recommended stretch range will vary depending on the bolt diameter, length and application, but will likely be in the 0.005- to 0.006-inch range. If the stretch is less, the bolt is probably experi- encing too much friction that is prevent- ing the proper stretch, (requiring lubricant on the threads). If stretch is excessive, the bolt may have been pulled beyond its yield point and is no longer serviceable. With high-quality performance rod bolts, when female and male threads are in good condition and the proper lube has been applied, and the torque wrench is properly calibrated, you can usually tighten the bolt to the specified torque value and then check for stretch, which will usually be within the recommended stretch range. I prefer to initially under-torque and then creep up to the desired stretch. It's more time-consuming, but it eliminates the pos- sibility of over-stretching. While an outside micrometer may be used to measure the rod bolt length, the most accurate method is to use a specialty Connecting rod bolt tightening is a critical phase of assembly. Precise rod bolt clamping force is required to provide the proper amount of bearing crush and radial pres- sure to obtain the required bearing geometry. (Illustration courtesy MAHLE) Examples of rod bolt stretch gauges. These are available from firms such as ARP, Goodson and GearHead. fixture that is outfitted with a dial indi- cator. Excellent examples of this gauge include units from GearHead Tools, ARP and Goodson Shop Supplies. GearHead's bolt stretch gauge features a heat-treated aluminum frame (with a very handy thumbhole), with a specially modi- fied dial indicator with sufficient spring tension to hold the gauge firmly to the ends of the rod bolt. The indicator can be rotated for right- or left-hand operation, and the lower anvil is adjustable to accom- modate various bolt lengths. Goodson offers a rod bolt stretch gauge featuring spherical points for consistent and repeatable readings, and can also be rotated for right- or left-hand operation. Also, ARP offers its own bolt stretch gauge, designed with 0.0005-inch increments with a heavy spring and ball tips. There is a Debate… … among some engine builders regard- ing the necessity of measuring rod bolt stretch, due to potential compression of the rod material as the rod cap is clamped to the rod. While this can occur, the use of a stretch gauge remains the best practi- cal method of accurately determining bolt load. Connecting rod bolts can be viewed as high-tensile springs. The bolt must be

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