Why you can’t reuse some automotive fasteners

My last repair job involved removing the drive axle from the steering knuckle. The shop manual warns not to reuse the axle nut, so I trotted off to the parts store for a new one. The parts store guy said he rarely sold the new nuts and told me to just reuse the old one. That’s not good advice, and it made me wonder if other DIYers were ignoring the shop manual warning as well.

 

I’m not talking about ordinary automotive nuts and bolts. I’m referring to the special nuts on major components like axle shafts and ball joints where a lost (or loosened) nut can cause accidents, even fatal accidents. These fasteners, called “prevailing torque nuts,” are shown here. They’re deformed into an elliptical shape during the manufacturing process (while they’re still hot). During the tightening process, the bolt or axle shaft stretches the nut into a round shape. Even after “rounding out,” the nut retains enough elliptical distortion to prevent loosening in high heat situations where thread-locking compounds normally fail.

 

Car makers use two types of non-reusable prevailing torque nuts: the center locknut style and the toplock variety. The center locknut version can be installed from either end, since the deformed portion is located in the middle of the nut. However, a toplock nut can only be installed one way, since the deformed end can’t engage the threads. Whichever style is on your vehicle, make sure you buy a new nut after removing the old one. The old nut will not hold the proper torque a second time. Don’t even think about “re-deforming” the old nut in your bench vise—it won’t work. New axle nuts cost about $3 and ball joint nuts usually less than $1. Your family’s safety is worth far more than that. 
Prevailing torque nuts
— Rick Muscoplat, Automotive Editor

 

For more information on nuts and bolts, read our How to Loosen Nuts, Bolts and Screws on FamilyHandyman.com. And also see this Nuts and bolts glossary: http://www.boltscience.com/pages/glossary.htm