Clamping a rod lengthwise will keep it from twisting, as long as the vise jaws don't put nicks in the beam section. I use soft copper jaws on anything that might be damaged by the serrations on the hard steel jaws. The precaution is about clamping a rod near the big end, and putting a lot of torque on the bolt at the top. That puts a twist into the beam because stoveblot rods are pretty flimsy where a twist is concerned. Twisted rods create an oddball angular wear pattern on the piston skirt, and a really strange-sounding knock!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!