Dear Car Talk:
Did power tools cause a lot of problems when first used to tighten lug nuts? We were with Mom in the 1972 VW Van and heard a loud clunking on the front passenger side. Mom pulled over, and we removed the hubcap to find one lonely lug nut in place -- the rest had sheared off! Was it an overtightening problem? -- Theresa
Probably. Unless it was an ex-boyfriend problem.
It is hard to control the torque (twisting power) of air-powered impact wrenches. And -- as you would expect -- most mechanics preferred to err by making them too tight rather than too loose. But as you and Mom learned, too tight isn't so good either. They can be tightened so much that they stretch the lug bolts themselves, and weaken them to the point where they can break off.
You're lucky you stopped when you did, because those VW Vans didn't run very well on three wheels. They regularly crossed the country on three cylinders, but not on three wheels.
Now there are things called "torque sticks" that help prevent overtightening. Each torque stick is rated for a certain amount of torque. It goes between the air wrench and the lug nut. And once it reaches its maximum torque, it starts to slip, which limits the amount of force that can be applied to the lug nut.
Torque sticks work well -- as long as the mechanic uses them. And as long as he doesn't keep hammering away at the lug nut after the stick starts slipping. Our guys often will put a torque wrench on a couple of random lug nuts after they're tightened, just to check that they're tightened correctly.
A torque wrench measures how tight the lug nuts are. So if a lug nut is supposed to be tightened to, say, 85 foot-pounds, and the torque wrench says it took 300 foot-pounds to remove it, the mechanic knows that something's wrong. But we see less of that these days, because we've actually switched over to battery-powered tools.
The great advantage of battery-powered tools, at first, was that you could reach into places that a bulky air wrench just couldn't reach. And you didn't have to worry about dragging the air hose into these little spaces along with you. But we also discovered that it's much easier to control the torque on a battery-powered impact wrench.
They're not perfect. If we come across a lug nut that's been really overtightened by some animal at another shop (or by one of the animals at our shop), sometimes the battery-powered wrench just doesn't have enough torque to get it off. Then we have to drag out the air wrench to remove it.
But we use battery-powered wrenches exclusively for lug nuts now. And you'll be glad to know we haven't sheared off a lug bolt in days, Theresa!