MEASURING ROD-BOLT STRETCH
Before dropping the pistons into the cylinders, the gudgeon pins and circlip were installed. Rich looked to be having great fun installing the pins — not. Before dropping them in, he did one final check to ensure the rod bolts were to spec. Rod bolts are the most stressed fastener in the entire assembly and get worked every rotation of the crank. The higher the rpm, the greater the load on the bolts, so achieving the correct amount of clamping force is integral to engine reliability — in other words, to not throwing a rod. If you over-tighten during assembly, you risk the bolt overstretching during high rpm, when the load on the bolt is maximized. Why?
Because a rod bolt is a sprung piece of steel, which achieves its maximum clamping force when stretched close to its maximum elasticity. But that does not mean you want to tighten to this point during assembly. You need to leave a buffer, because, when under maximum load, at high rpm, the bolt will stretch further and could stretch beyond its maximum elasticity. Once a bolt has reached this point, it will no longer return to size and, instead, will remain permanently stretched. It is for this reason that measuring the rodbolt stretch is preferred to torqueing the bolts, as the torque method measures the friction, which is affected by assembly lube, etc., giving you no indication how close the bolt is to over stretching.
Rich first used a torque wrench to 94.9Nm, and then an ARP rod-bolt gauge and checked them before installing. It’s a good thing he did that this time, too, as he found two of the brand-new bolts from ARP to be well outside the 0.004 to 0.007 stretch. Not wanting to risk it, he ordered and installed a full new set of higher-grade ARP2000 bolts.