The humble lug nut is a critical player in vehicular safety. Too loose and your wheel could part company with the axle and roll on down the highway on its own — yikes. In fact, recall that some Ford vehicles actually left the factory with loose lug nuts in 2023. At the other extreme, ham-fisted mechanics using an impact gun can overtighten lug nuts. That can cause the wheel stud to stretch or break, not to mention making a wheel impossible to remove by the side of the road with hand tools if you happen to suffer a flat tire.
Now we can add another potential pitfall to the simple wheel-fastening device — swollen lug nuts. Swollen lug nuts are a phenomenon that afflicts two-piece lug nuts, sometime referred to as capped lug nuts. Two-piece lug nuts consist of a regular tough steel lug nut covered with a thin layer of steel, stainless steel, or aluminum sheet metal. Basically, it’s a cosmetic cover that’s installed over the actual plain steel lug nut to give the appearance that the entire lug nut assembly is polished or chrome plated.
Moisture or rough handling is the culprit
Where problems start to form with two-piece lug nuts is when moisture gets trapped in between the decorative cap and the actual lug nut. Although it’s something that can occur in all climates, the sometimes-toxic ice melting agents used to salt the roads in snowy climates can accelerate hidden rust developing between the two pieces. That rust causes the steel lug nut to expand, which pushes against the decorative cover and causes it to swell to larger than its original diameter.
The conditions for swelling in two-piece lug nuts can become more likely to occur if the polished or chrome cap becomes loose, which increases the potential for moisture ingress. I’ve even seen a cap completely detach from the underlying lug nut. Frequent exposure to impact wrenches can loosen the sheet-metal lug nut cap. In addition, using a low-quality or improperly sized socket — such as substituting an SAE socket for a similarly sized metric one — can damage the cap.
It should go without saying that capped lugs don’t outwardly look like two separate pieces, which is the whole point. Most folks just see a beefy polished or chromed lug nut without considering that it’s actually a veneer. Car owners may be required to do a little internet sleuthing or make an inquiry with their dealership’s service department to determine if their vehicle uses this style lug nut or not. Alternatively, you can simply give your lug nuts a thorough visual inspection for signs of corrosion or swelling. It’ll be particularly noticeable on the six flat surfaces on the sides of the lug nut.
It’s annoying, but not life-threatening
If you’ve discovered swollen lug nuts on your vehicle, the first issue could be removing them. After all, if a 20 millimeter lug nut has swelled to 22 millimeters, the original lug wrench or specified socket size will no longer fit properly. In some cases, it may be possible to hammer the original lug wrench or socket onto the nut, or step up one or two sizes until a right-sized socket is located.
Of course, access to a variety of socket sizes and other tools may not be an option if you’re stranded by the side of the road with a flat tire with only your car’s factory tool kit available. It’s also possible that the lug nut swelling might not occur uniformly. That means that the lug nut cover may be distorted and no longer totally hexagonal. Removing swollen lug nuts is so difficult in some cases that even roadside assistance pros like AAA haven’t been able to get them unstuck to change a tire. Instead, vehicles required towing to a repair shop.
Once removed, swollen or overly corroded lug nuts can be replaced with fresh original equipment lug nuts from the vehicle’s manufacturer or a lower cost reproduction part. In either case, the swelling may reoccur after a certain period of time. Alternatively, you can source higher quality one-piece solid aftermarket lug nuts in chrome or a myriad of other finishes. If going with aftermarket lug nuts, note that they may require a different size lug wrench than the original capped lug nuts, which means that the tool in your spare tire kit will no longer fit and should be replaced.
