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continued from Part 2
Considering the anticipated forces that a set of rod bolts will encounter during high-performance engine use, it doesn’t make sense to compromise by tightening with torque alone.
Two examples of rod bolt stretch gauges. A variety of manufacturers offer these tools. The bottom anvil (stationary pointed pin) is adjustable to accommodate bolt length. The tool on the left is adjustable by loosening a small set screw and sliding the anvil. The tool on the right features an adjustable sliding base plus a fine tune adjustment of the anvil pin itself.
High-performance rod bolts feature a dimple at each end. This provides a convenient location for a stretch gauge.
This stretch gauge features a very handy “finger hole” design for easier gauge handling. All quality rod bolt stretch gauges accomplish the same task. Try a few different styles to determine which is more comfortable for you.
Carefully locate the stretch gauge to insure that the top and bottom gauge anvils are centered in the bolt head and top dimples.
If you plan to cycle new rod bolts, clamp the rod in a dedicated rod vise (the sample shown here is from GearHead Tools). This will protect the rod from damage while providing a stable platform during bolt tightening/loosening.
An example of tensile overload. This rod bolt stretched and “necked down” prior to rupture. One fracture face features a cup shape and the other face forms a cone shape. This type of failure indicates that either the rod bolt was under-tightened (not achieving proper preload) or was over-tightened beyond the bolt’s yield point. (photo courtesy ARP)
An example of torsional shear or twisting. Threaded fasteners are not normally subjected to torsional stress. However, when galling takes place between the male and female threads (caused by incorrect lubricant, the absence of lubricant or if the bolt was “cross threaded”), this type of failure is likely to occur during assembly. (photo courtesy ARP)
If, for whatever reason, you decide to tighten your rod bolts with torque instead of by stretch, make sure that your torque wrench is properly calibrated. Apply rotational force smoothly and slowly. Also, regardless of which method you use, be sure to lubricate the rod bolt threads and underside of the bolt head with the lubricant recommended by the bolt maker. Moly provides reduced friction as compared to oil, but pay attention to torque specifications if you plan to tighten with torque since specified torque values will differ between the use of oil versus moly.
Shown here is the tip of a performance rod bolt. Notice the dimple. The dimple feature on both head and tip are intended to accommodate a stretch gauge.
The forces exerted on the rod big end during engine operation can easily result in bore distortion and bolt bending, however minute. This is a major reason that stretch-tightening is often recommended since this tightening approach allows you to accurately determine if the rod bolts are preloaded just beyond the rod’s loaded force.
Both reciprocating loads and big-end flex have a direct impact on rod bolt durability as the rod cycles between zero to maximum loads. (illustration courtesy ARP)
An example of cyclic fatigue caused by a manufacturing error. Some of the high-strength “quench and temper” steel alloys in popular use by some manufacturers are subject to hydrogen embrittlement and, in essence, may fail due to “hard spots” within the structure. (photo courtesy ARP)
An example of cyclic fatigue caused by cracks that propagated from a rust pit, otherwise known as stress corrosion. Bolt materials such as L-19, H-11, 300M and Aeromet 100 are particularly susceptible to stress corrosion so must be kept oiled at all times and never exposed to moisture, including body sweat. Inconel 718, ARP 3.5 and Custom age 625+ are immune to both hydrogen embrittlement and stress corrosion. (photo courtesy ARP)
An example of cyclic failure caused by improper installation preload. When a fastener is insufficiently preloaded during installation, the dynamic load (forces generated during engine operation) may exceed the clamping load, resulting in cyclic tensile stress and eventual failure. The photo on the left shows the remainder of a bolt in the rod. The arrows on the left and right indicate the location of a cut that was made to free the bolt. The third arrow at the bottom shows the origin of the fatigue crack. The photo on the right (magnified close-up) shows the origin of the failure and the telltale “thumbprint” or “beach mark” (bottom). This fatigue crack radiused outward until bolt failure took place. (photo courtesy ARP)
Whenever installing a set of rod bolts with the stretch method, it’s a good idea to create a rod bolt monitoring chart. Identify each rod location (cyl 1, cyl 2, etc.). Within that rod’s box, identify the inboard and outboard rod bolts (relative to installed position). Record the relaxed length and the installed length. During the next engine teardown, you can refer to the chart as you once again measure each rod bolt’s relaxed length. If a bolt has permanently stretched 0.001″ or more in a relaxed state, the bolt must be discarded and replaced. When tightening connecting rod bolts (either as the final method or as part of a torque/angle method), stabilize the ratchet head and pull with a slow, steady motion. Never jerk or pull quickly, as this will result in inaccurate torque value, possibly pulling past the desired point. (illustration courtesy ARP)
Tags: ARP, BOLT STRETCH, CONNECTING RODS, GEARHEAD TOOLS, GOODSON, ROD BOLT, ROD VISE, RODS, STRETCH, STRETCH GAUGE, TORQUE
