Caution With Battery-Powered Impact Wrenches
Sep 08, 2018
Back in the day, we didn't use torque wrenches on day-to-day repairs because we could "feel" when we had fasteners properly tightened. (Engine cylinder head bolts, connecting rods and other precision assemblies always required, and still require, torque wrenches, of course.) During common repairs it was safe to tighten anything larger than a 1/2" bolt as tight as you could get it, when you were tightening it by hand. When hand-tightening smaller bolts, like a 5/16," it was common to use only one finger for the final pull, to reduce chances of over-tightening.
Fast-forward to today. The advent of battery-powered impact wrenches has made it too easy to over-tighten nuts and bolts. Early battery-powered tools were somewhat underpowered, and it was relatively safe to "let it rattle" to ensure fasteners were adequately torqued. But the latest generation of battery-powered impact wrenches, with up to 700 lb/ft of torque, make it a risk to rattle smaller nuts and bolts.
I've been paying attention to tech manuals lately, checking out the recommended torque for 8 mm, 10mm and other common small fasteners, and they often don't need hundreds of pounds of final torque. I'm not saying every bolt needs to be checked with a torque wrench, but I am saying that if you watch a professional mechanic use a battery-powered impact wrench, they don't just pull the trigger and let the gun rattle anymore. With the power of the new guns, it's good to be a little light-fingered when tightening any fastener 1/2" or smaller. When in doubt, spin the fastener into place with the battery-powered gun, then hand-tighten the final pull to prevent problems.