2023 ARP Catalog

39 The Stretch Gauge We highly recommend using a stretch gauge when installing rod bolts and other fasteners, where it is possible to measure the length of the fastener. It is the most accurate way of measuring clamp load of any bolt. Simply follow manufacturer’s instructions, or use the chart on pages 54-57 of this catalog for ARP rod bolts. When using a stretch gauge it’s best to measure the fastener prior to starting and monitor the fastener overall length during installation. When the bolt has stretched the specified amount, the correct clamping force has been achieved. We recommend that you maintain a chart of all rod bolts and make a note of the fastener length prior to installation and after any disassembly. If there is a permanent increase of .001˝ or more in length, there is a deformation and the bolt should be replaced. A sample stretch monitoring chart is located on page 53. Using A Torque Wrench There are a number of things to consider when using a torque wrench. The “friction factor” changes from one cycle to the next. That is, friction is at its highest value when the fastener is first tightened. Each subsequent time the fastener is torqued and loosened, the amount of friction reduces. Eventually the friction levels out and becomes fairly consistent for all following repetitions. Three basic elements that contribute to the friction factor: 1. Most importantly - the fastener assembly lubricant 2. The condition of the threads 3. The surface finish of the fastener Because of these variables, a phenomenon known as clamp load scatter occurs. Clamp load scatter is the variation in clamp load a fastener generates due to fluctuating levels of friction from one torque cycle to subsequent cycles. It’s not uncommon to see clamp load scatter in the range of 4,000-8,000 pounds between the first and tenth pull on a new fastener depending on the lubricant used. The Lubricant Is The Key A major factor that influences friction in a fastener is the lubricant used, and therefore influences the torque required for a particular installation. One of the most overlooked aspects of choosing a fastener assembly lubricant is…the lubricant’s ability to stabilize friction inherent in all high performance engine fasteners. As discussed earlier, friction is at its highest point when a new fastener is first tightened. This friction inhibits the fasteners ability to achieve the required clamp load on the first several cycles. In fact, ARP’s in-house Research and Development department has proven that new fasteners using motor oil and other commonly used lubriProper Fastener Retention FASTENER TECH