I understand that all fasteners stretch when tightened down and there is a point where it is weakened by over stretching it.. My question is how does that tool work?

 

On rod bolts you put the guage on the bolt.The bolt has dimples that the ends of the guage fits into.You set the guage at zero which is the outstretched length.Carefully remove the guage.Now I usually torque the bolt to to the recommended spec. with the recommended lube.Reinstall the guage.The number on the guage is how much it stretched.If the number is correct your all done with that bolt.If its stretched to much I loosen the nut,rest the guage to zero (making sure the bolt didn't take a permanent stretch more than a couple tenths of a thou.) and retorque it to 5 or 10 pounds less and measure it.If its not stretched enough I go up 5# and check it again.Recommended stretch is usually with in 5 or 10 pounds of the recommended spec.It is a little time consuming but its truly the right way to go.It gets easy after a couple engines.
Glenn

 

ok so say you are using arp rod bolts and after you torque them and loosen them and measure how much they have streatched, if they have been over stretched they are useless correct? Can you take pics of the process like a Tech Article

 

Most aftermarket rod bolts (ARP,Crower,SPS,Oliver,etc...) have dimples in the end of the bolts for using a stretch guage.Yes,ARP recommends replacing the bolts if they stretch more that .001" over their original length (untorqued).ARP Honda LS bolts will stretch .006" many times before taking a permanent set off +.001" over original length.The torque numbers that a manufacturer gives you are an easy way to achieve the bolt stretch they are looking for.Here is a picture or a guage in use. http://www.bhjinc.com/1products/0-landing.htm
Glenn