53 WARNING: Cancer and Reproductive Harm - www.P65Warnings.ca.gov. s l California Residents WARNING: Cancer www.P65Warnings.ca.gov. Installation Stretch versus Cyclic Stretch Proper installation of a rod bolt with the correct pre-load is essential in preventing early failure due to cyclic fatigue. In a rod bolt that is installed with the correct pre-load, the pre-load in the bolt is higher than the highest load seen during engine operation. This means that the rod bolt does not experience any additional stretch above the installation stretch. However, when the rod bolt is installed with a pre-load that is too low, every revolution of the crankshaft causes the bolt to stretch more than its installation stretch. As the piston moves past top dead center, the bolt relaxes to its installed stretch - this is cyclic stretch. This cyclic stretch causes the rod bolt to fatigue prematurely leading to rod bolt failure with its ensuing consequences. This is why it is so critical to install rod bolts to the specified stretch (or torque) values. Measuring Rod Bolt Length is the Key To Preventing Failure Measuring rod bolt installation stretch with a stretch gauge is the best method for installing rod bolts (although a micrometer will work in some cases). However, the key to determining whether a rod bolt can be re-used is measuring and tracking the pre-installed length of each rod bolt by location and comparing it to the length at tear down. Using a chart, like the one on the page 54, makes this job a lot easier. If the tear down length of the rod bolt is more that 0.001˝ longer than the pre-installed length, then the bolt should be replaced. This means the bolt has taken a permanent stretch past its yield point - either from cyclic loading or a significant engine event - and should not be trusted. Installation Friction Variance Problem Solved With ARP Ultra-Torque ARP created ARP Ultra-Torque Fastener Assembly lubricant to solve the problem of pre-load variability in fastener installation. This problem is especially critical in applications where the torque method must be used, as this variance can result in rod bolts that are installed with to little pre-load that allows cyclic stretching. ARP Ultra-Torque equalizes installation friction from fastener to fastener and from one installation cycle to the next. This means that the specified torque value results in the correct installation pre-load stretch. This completely eliminates the old solution of cycling the fasteners multiple times to even out the installation friction. See page 171 for more information on ARP Ultra-Torque. Other Fasteners Are Not as Critical as Rod Bolts In other types of bolted joints, this careful attention to tightening is not as important. For example, flywheel bolts need only be tightened enough to prevent them from working loose. Flywheel loads are carried either by shear pins or by side loads in the bolts; they don’t cause cyclic loads in the bolts. Connecting rod bolts, on the other hand, support the primary tension loads caused by engine operation and must be protected from cyclic stretching. That’s why proper tightening of connecting rod bolts is so important. See pages 54-57 for recommended stretch and torque values. revised 10/2025 A rod bolt stretch gauge is one of the most important tools a serious engine builder can own. It is valuable in properly setting up a rod for resizing, obtaining the proper clamp load when installed. See page 172 for more information. The Importance of Proper Rod Bolt Stretch/Torque ROD BOLTS
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