Maintenance Q&A
Studs and lugs

By Paul Abelson, senior technical editor

Q. At a little over 600,000 miles, it was time to change tires on my 2006 Volvo VN again. I went to my usual tire dealer. Despite a change in ownership, they still carry the same brands and have most of the same people working there.

They put on the new tires OK. But on the bill there were charges for six sets of new studs. I didn’t order them, and no one asked me if I wanted them. They never put any on before.

Is the new ownership telling them to up the sales? I understand the charges for mounting and balancing, but why new studs?

A. It seems to me the owners and managers are being cautious and responsible. When you had your tires changed before, the dealer probably checked your studs and found them in serviceable condition. This time, they may have been damaged and not serviceable. If your dealer sent you out on the road with defective or damaged studs, he could have been liable if you lost a wheel or had a crash.

You mentioned you were in a hurry and didn’t question the bill until the next week. By then, the dealer assumed all was well and he sold the old steel studs for scrap. It’s always wise to glance over the invoice at the time the work is picked up. Even if you can’t go over it in detail, you can question things and ask the dealer to save the parts for examination when you return.

A number of things can damage wheel studs. Most often, it’s corrosion, improper installation methods or over-torquing. Whenever wheels are off, it’s a good idea to visually inspect studs. First, look for corrosion. If any is present, discard the stud.

Studs generate their clamping force because of the elastic properties of steel. Properly torqued, the threads on the nut engage those on the stud and stretch the stud. As long as the stud has not exceeded its elastic limit, the stud wants to return to its original shape. That pulls the nut and the head together, clamping whatever is between them. In this case, it’s the wheel and brake assembly. It’s how all threaded fasteners work.

Problems arise when either too much or too little torque is applied to the nut. If too little, there will not be enough force to hold things in place. Vibration will loosen the nut further, and the wheel will rub over the threads, enlarging and distorting the bolt holes. Eventually, the wheel will loosen, crack or both. Too little clamping force may show up as damaged threads on the stud.

When too much torque is applied, threads stretch the stud beyond its elastic limit. It can no longer spring back to clamp the parts together with sufficient force. In many cases, the stud permanently elongates, increasing the distance between threads while narrowing the stud’s diameter.

Elongation too slight to detect with the naked eye may be great enough to ruin the stud. It can be found several ways. The first is with a thread gauge. The gauge should sit evenly in the threads. You can also lay the suspect stud against a new one to see if the threads match.

A straight edge will also show any distortion or elongation of the threads when laid across the highest points of the threads. If any threads don’t make contact, discard the stud. Another way is to run a nut down the stud. It should feel the same all along its path. Any binding or interference is a sign of distortion.

If possible, observe how the shop mounts wheels. If not, leave instructions to do final tightening by hand with a torque wrench. Compressed air-powered impact wrenches are excellent for removing wheel nuts, but not for tightening them. The errors made with air wrenches are almost infinite, with most leading to over-torqued and stretched fasteners, and eventually to failures.

Air wrenches should be calibrated regularly, then set to a torque value below that needed to firmly secure wheels. They should only be used to snug-up nuts. Final torque should always be applied by hand, using a recently calibrated, properly cared for torque wrench.

For hub-piloted wheels using M20 X 1.5 nut thread, 280 to 330 lb-ft of torque should be applied to threads oiled with no more than two drops of SAE 30 weight oil placed on the stud’s first two threads. For similarly oiled M22 X 1.5 wheel nuts, 450 to 500 lb-ft should be applied. For 7/8 to 1/4 nuts, torque should be between 350 and 400 lb-ft.

When using wheel nuts with rotating flanges, place only one drop of SAE 30 oil between the flange and nut. Do not over-oil and do not get oil on the face of the flange that bears on the wheel. You will get false torque readings.

Far too many shops are in too much of a hurry. Many use only one torque setting for all wheel work, regardless of size. They don’t check torque, relying instead on torque-limiting adapters or “torque sticks” to absorb some of the torque delivered by an impact wrench. These are grossly inaccurate.

Since wheel tightness is so important, mechanics may “hit it again” for a few more seconds. That’s a sure way to guarantee stretched fasteners.

Rather than overselling or billing for items you didn’t request, your dealer might be thorough and conscientious. He may have saved you from a catastrophic failure and himself from any related liability.

The answer was compiled from practices and procedures found in TMC Recommended Practices. LL

July Digital Edition