Anytime you tighten a rod bolt in the rod, always support the big end of the rod across the cap to prevent tweaking the rod. This is a Bill Mitchell Hard Core rod vise, but several other companies offer their own versions.
ARP generally sets the stretch figure at 75 percent of the bolt's yield or elastic limit. This creates the most load with still plenty of safe "headroom" between the stretch figure and the bolt's limit. One advantage to measuring bolt stretch is that this figure is unaffected by whatever lubrication you place on the threads. This is not the case when using a torque wrench.
Stretch vs. Lube
When you tighten a rod bolt nut using a torque wrench set to a specific spec, the wrench is measuring a resistance to movement. This resistance is partially the result of bolt stretch, but the majority of effort required to torque the bolt comes from friction. This friction results from the threads moving against one another as well as the rod bolt nut against the rod cap. This friction changes based on the type of lubricant used. For example, friction will be greatest with no lube and slightly less if oil is applied to the threads and the bottom of the nut. It will be reduced even more if you apply some of ARP's bolt lube to the threads and the face of the nut. The lube's friciton "index" is referred to as its friciton coefficient.
When tightening a rod bolt in the engine, the best procedure is to use a box-end wrench. We use a cheater bar over the end of the wrench for additional leverage. Be careful not to disturb the rod bolt-stretch fixture when tightening. If so, loosen the bolt and start over.
Perhaps you can see where we're headed with this little exercise. If you were to apply the same torque to a fastener with no lube and then use different types of lube, the stretch on the bolt would increase with the ability of the lube to reduce friction. This is why it is extremely difficult for a fastener company to give an accurate torque spec that will properly stretch the bolt because there are so many lubrication friction coefficient variables. If you must torque a rod bolt, ARP offers a torque spec with its lube.
Better Bolts
ARP offers three different styles of rod bolts for most applications with increasing tensile strength for each version. The standard ARP high-performance 8740 alloy chrome-moly steel bolts offer a tensile strength of roughly 200,000 psi and are good for most all street-oriented applications. The Wave-Loc ARP bolt is the next level up using the same heat-treated 8740 steel but has patented symmetrical waves incorporated into the bolt shank that fit snugly into the rod without the knurling that can cause sharp edges and a stress riser that can cause a crack to eventually form.
ARP's Assembly lube is a special moly-based blended lubricant designed to be a consistent lubricant to minimize variations when torquing rod bolts. The lube comes in containers from a 0.5 oz to a 1-pint bottle.
ARP's best bolt is the Pro series Wave-Loc that uses higher-strength ARP2000 material with a tensile strength of approximately 220,000 psi. This higher-tensile-strength bolt is offered as an upgrade for ultimate competition engines that experience ultra-high rpm use such as Pro Stock or endurance road racing.
Now you know a little more about the stresses imposed on a connecting rod. For example, a 434ci small-block with a 4.00-inch stroke will generate a massive 3,200 g's at TDC at 6,500 rpm. Multiply the weight of the piston and the small end of the rod by this g-force and you can get an idea of the tension exerted on a connecting rod and its two bolts. Think about that the next time you want to just torque your rod bolts instead of stretching them the way you should.
It's also a good idea to measure the overall length of each rod bolt in the engine before torquing. Then when the engine comes apart, you can measure the bolts again. If the bolts all measure the same, you can safely reuse them. If one is stretched even just 0.001 inch, immediately replace the bolt. | 
Any time that a bolt is replaced in a connecting rod, the big end must be resized to prevent problems. This is because pressing a new bolt in place can easily change the clamp characteristics and drastically affect the rod's concentricity. |
ARP's standard rod bolts (left) are more than sufficient for most street engines and even mild race engines. ARP's next step up is the Wave-Loc bolt (right) that employs the concentric undulations in the shank that reduce the risk of creating a crack from stress risers. | |