It requires a special fixture that clamps the rod into it and compares the alignment from end to end, both for bend and twist. It used to be a standard check when rods were reconditioned as part of any good engine overhaul, but newer engines have gone to short, very rigid rods and most "machinists" have never heard of the procedure. Even the guy who does my crankshaft grinding had never heard of straightening rods when I brought up the subject recently.
Jerry
