i dont understand the need to stretch rod bolts. no other bolts are ,so what makes rods so special
Every bolt is stretched - that is what actually retains the bolt. Rod bolts are just one of the few applications where it's easy to measure the actual stretch. Measured torque is the next best thing when you can't measure stretch directly. Since most bolts are made to one of a couple different industry standards, the engineering has been ballparked so we know that on a particular grade and size bolt that XX torque will = a certain stretch.
Some of the larger MTU engines I've worked on the main studs are "torqued" by using a hydrualic cylinder to stretch it and the "nut" is just spun down to the saddle by hand and the cylinder released. The first time you do one it's a bit unnerving watching a stud that bit visibly stretch! There are other engines that employ a similar process for certain fasteners. Also not uncommon in certain crane applications as well.
You don't need to measure stretch directly on rod bolts, it's just more accurate.