I can offer a few observations. The biggest cause by far of broken studs is failure to use a click-type torque wrench on the nuts. A car like yours will typically call for around 90 - 100 foot pounds. It is real easy to over-tighten them which stresses the nuts and the studs. Using the torque wrench also insures they're all tightened exactly the same amount. That makes the clamping forces equal all the way around and prevents warping the brake rotor. All professionals use torque wrenches. Failure to do so could get them fired.
Look at the rounded friction surfaces where the nuts and wheel contact each other. Once the nuts come loose on a wheel, that wheel and the nuts must be replaced because it is going to continue to do that. The friction surface will become rough and worn and won't make full contact. That will let the nuts work loose again very quickly.
Some people put grease on the studs to prevent rust. A tiny little bit is okay, but then the nuts must be run on by hand, ... No air tools. The centrifugal force when using air tools causes the grease to build up ahead of the nuts and spray onto the friction surface. There must be absolutely no grease between the wheel and nuts. That friction surface is what keeps the nuts tight.
If you used anti-seize compound anywhere on the studs, get that completely washed off. That stuff has its uses and wheel studs is definitely not one of them. Mechanics will be fired for doing that.
Any type of spacer between the wheel and hub is a very bad idea. The forces from the wheel flexing will crush what's stuck in there and allow the wheel to work loose. The wheels already flex every time they go around. With spacers in there, there's no way the wheel can make full contact and it's going to really flex a lot more. That will work the nuts loose repeatedly and could lead to a fracture and broken wheel.
Friday, May 17th, 2013 AT 9:58 PM