Like anyone I was up bright and early to place the lower end of our monster 996TT motor together. Part of that process is placing the rods on the crankshaft. In most cases of factory rods engine-builders have been tightening rod bolts with a torque wrench and getting away with it. But in today’s high-performance world where we push engines to make a ton more power and spin at higher engine speeds, merely tightening rod bolts with a torque wrench is no longer the best way. In order to know why rod bolt stretch is a more accurate way to install a bolt, we have to get into a little bit of fastener basics.
A fastener works like a coil spring. As you tighten a bolt, it will stretch and generate a clamp load. The key to proper tightening of a fastener is to torque it until just slightly less than the bolt’s elastic limit. A bolt will stretch slightly as it is tightened. If you tighten the bolt too much, it will stretch beyond its elastic limit. If you measure a bolt’s overall length before you overtighten it, then again when you release the preload, the bolt will be slightly longer. This is similar to overstretching a coil spring. It does not return to its tightly packed position because the steel has been stretched and over-yielded. This is the bolt’s yield point, where it is permanently deformed. It’s the point just before it comes apart. The bolt’s ultimate tensile strength is the maximum stress imparted on the bolt before it breaks.
This applies to all fasteners, but it’s especially critical with rod bolts because they’re the most highly stressed fasteners in an internal combustion engine. With every revolution, the crankshaft yanks on the piston and rod assembly to pull it away from top dead center (TDC). The rod journal pulls on the rod cap, which tries to stretch the rod bolts. This stress becomes greater as engine speed increases since this load increases geometrically with rpm and forces the rod out of round, bending and fatiguing the bolt.
The key to keeping the rod cap on the rod is the amount of load created with the rod bolts. If the load created by the bolts is greater than the tension created by the crankshaft rod journal trying to pry the cap off the rod, then the engine will stay together. If the bolt is not properly preloaded (understretched) then the high-rpm tension is enough to stretch the rod bolts a very tiny amount with each revolution. This high-speed cycling of the bolt is similar to bending a paper clip back and forth until it breaks. That’s obviously something you want to avoid.
Torque vs. Stretch
The torque spec applied to any particular fastener is merely an estimate of the twisting force required to achieve the correct amount of preload or clamp load. Many times this is the only way to apply fastener load because the bolt threads into a blind hole like in the cylinder block. One advantage to the rod bolt is that both ends of the bolt can be accessed. This allows you to use a rod bolt stretch gauge. This is a specialty tool sold through companies like ARP that will accurately measure the amount of bolt stretch.
The procedure is actually quite simple. Once the connecting rod and cap are installed on the crank, start a nut on the rod bolt, slip on the appropriate-size box-end wrench, and then install the stretch gauge. Most high perfromance connecting rod bolts have a small dimple placed on both ends of the bolt that accurately position the rod bolt gauge pins on the bolt. Next, zero the gauge on the relaxed bolt. Then you carefully tighten the rod bolt until the gauge reads the appropriate stretch amount. For example, on this Porsche rod bolt the stretch was to be 5-7 thousands with no more than 65 ft lbs of TQ. We know that at 6.5 thousands we are at 58 Ft lbs and well within the meat of the spec.
Last but not least the bed in which the crank will lay…More on that later.