Rod bolt stretch is the measured elongation of a connecting rod bolt as it is tightened, read with a stretch gauge and taken to the bolt maker's spec in thousandths of an inch, because elongation is clamp load.
The stretch method takes friction out of the equation. ARP calls stretch measurement the most accurate way to set preload and notes it is practical mainly on rod bolts, since they are one of the few fasteners you can reach from both ends. Measure free length with a stretch gauge, write it down, then tighten until the gauge reads spec rather than until a torque wrench clicks. The number is part-number specific: ARP’s own instruction sheets list 0.0050–0.0055 in for one rod bolt kit and 0.0070–0.0075 in for another. Pull the sheet for the bolt in your hand. ARP also says to replace any fastener whose free length has grown 0.001 in or more.
Stretch is the only clamp-load number you can actually verify, and an under-clamped rod bolt fatigues until it lets go and takes the block with it.
Read the full post:
When to Replace Rod Bolts and Valve Springs: The Parts That Fail Without Warning