There was a time when wheel lugs were very basic. Most were 7/16 or half inch diameter and all worked about the same. Now there are dozens of metric and inch sizes with several different thread pitches. The way they hold and center the wheel is also different and can be dangerous if the wrong application is used.
Original equipment wheels are designed hub centric
The diameter of the hub tightly fits the inner diameter of the wheel. This very accurately centers the wheel. Some aftermarket wheels are designed to fit multiple applications. The diameter of the center hole is large enough to fit over the largest hub they accommodate. These wheels rely on the proper lug nut to center the wheel. If the wrong style lug nut is used, the wheel will not be centered and may come loose. Wheels that vibrate, even after repeated attempts at balance sometimes have this problem. (please click image for closer view)
Even with original equipment wheels several designs of lugs have been used. There are conical seats, which are slightly rounded. There are also wedge based lugs, flat faced lugs and mag style lugs to name a few. With the wedge base, an angular face matches the same type seat in the wheel. Interchanging conical and wedge base lugs can cause them to break and cause the wheel to come off.
With the flat faced lug, used on Ford products, the lug provides no centering at all. The flat face holds the wheel tightly against the hub. The hub or axle flange centers the wheel.
With the mag style lug, the shoulders of the lug fits down into holes of the same size in the wheel. A thick washer is also used to aid in clamping the wheel down. In non-hub centric applications these type lugs are often used in an attempt to center the wheel.
Matching the proper type lug to the wheel is imperative and mis-matches can cause major safety issues. It is also imperative to match the lug thread diameter and pitch with the stud. Some applications will thread partially on, but will quickly destroy the threads.
When tightening lugs, regardless of the style, a torque device should be used. Lug studs operate as very powerful springs. As the lug is tightened, the material of the stud is stretched. This stretching provides the clamping force that holds the wheel tight. Too much torque can permanently stretch, weaken and break the stud.
Excessive and improperly applied torque can also warped the hub. When this occurs, the brake rotor, which also fits over the hub can be warped. Over tightening can also collapse the hole for the drive axle, on front wheel drive applications. When this happens, future removal of the axle can become impossible.
Lug torque varies considerably between vehicles. The maximum torque for one vehicle may be insufficient to hold the wheel on another. Professional technicians always consult the manufacturer’s service data. The owner's manual of your vehicle will usually provide lug torque, listed under specifications. Lugs should also be tightened in a specific sequence. This allows the load to be spread more evenly. Most technicians also feel tightening lugs in steps is beneficial. For instance, if the specification is 90 foot pounds, they would all be tightened to thirty foot pounds, then sixty and finally ninety.
Wheel Lug Torque Specifications
If no specification is available, the following chart based on lug diameter, is a rough guide. This should never be used to replace manufacturer's specifications and is only an approximation. Lugs should be tightened to proper specification as soon information is available.
Stud Diameter
Lug Torque
10 MM Stud
50 Foot Pounds
12 MM Stud
80 Foot Pounds
14 MM Stud
90 Foot Pounds
7/16 Inch Stud
75 Foot Pounds
1/2 Inch stud
85 Foot Pounds
9/16 Inch Stud
140 Foot Pounds
5/8 Inch Stud
160 Foot Pounds
Lastly, it is important to realize the specification for lug torque is a dry specification. Lugs should NOT be lubricated. Torque is a resistance to turning that relates to tightness. Lubricating lugs will drastically reduce this resistance to turning and the same torque will stretch the stud far more.
Like most things on automobiles, lug nuts appear deceptively simple. In fact wrong procedures can result in damage and worse. A few simple precautions can help prevent a very dangerous situation.
