2004 GMC Sierra Replacing calipers and rotors

Tiny
JJLEIGHTON
  • MEMBER
  • 2004 GMC SIERRA
  • 6.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 160,000 MILES
When I put the new calipers on over the new rotors the rotors become uneven and crooked.
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Tuesday, June 16th, 2015 AT 12:14 PM

1 Reply

Tiny
CARADIODOC
  • EXPERT
You have to list the exact model and size so I can look up which brake system you have and the style of parts used. I can't find anything with a 6.7L engine. Are these rotors on the front or rear? Dual rear wheels?

Do you have "captive" rotors, meaning you have to remove the wheel bearing assembly, then bolt on the rotors on the backside?

When professionals do brake work, they spend the most time cleaning parts and preparing them for assembly. Actually putting the pieces together takes up a very small percentage of their time. One of the most common causes of what you described is caused by reusing old rotors, which is not the case here. You'll see three small access holes in the hub where the rotor mounts on. Water splashes up there and forms rust spots on the backside of the rotor. Those spots have to be ground off. If they are not, the rotor will not sit squarely on the brake lathe and a warp will be machined in.

Even if the rotors machine true, or are already true and are not machined, there are five, six, or eight different ways to install it on the hub. If it is installed in a different orientation that how it was before, those rust spots will be caught between the hub and rotor and prevent it from sitting squarely. The rotor and the wheel will wobble sideways as it rotates.

The next thing is the hub must be wire-brushed on the contact point where the center hole of the rotor sits, then there needs to be a light coating of high-temperature brake grease applied. Rust builds up in that area and can break chunks off, when the rotor is removed, that get stuck between the rotor and hub. Cleaning the rust off prevents that, and the grease helps the rotor sit straight when it is clamped down by the lug nuts.

The lug nuts must be properly tightened with a click-type torque wrench. All professionals do that to insure the clamping forces are uniform to prevent the rotor from warping later, to prevent peeling the threads, to insure the nuts don't come loose, and to insure anyone can get the nuts off to change a flat tire. Over-tightening the lug nuts will damage the threads, and often leads to one of them spinning but won't come off. The mechanic trying to get the wheel off is blamed for the damage, but it was actually caused by the last person who put them on and over-tightened them.

If you have captive rotors, those are held in place by tightening the lug nuts. It is not uncommon for the rotor to appear to wobble until those studs are tightened to seat the rotor to the hub. The nuts should be rechecked and torqued again after driving a few miles.

Also look on used rotors where the rust ridge forms just outside of the outer circumference of the hub. Rust chips can break off there too and get stuck between the rotor and hub. That rust and scale should be scrapped out as much as possible.

It also is important to remember to never, ever use anti-seize compound on lug nut studs. That stunt will get a mechanic fired on the spot. There is no way the nuts will stay tight. The must be no grease or other lubricants applied to anodized studs. Those studs have a light blue, light yellow, or silver electric plating that is a lubricant already. For those of us who DO use grease on studs, that must only be a very light film of axle grease on the threads to prevent rust, but then the nuts must be spun on by hand, not with an air impact wrench. Spinning the nuts with air tools will cause that grease to build up ahead of the nut and be spun out by centrifugal force onto the smooth friction surface. Those friction surfaces where the nut contacts the wheel must remain perfectly smooth, round, and dry. That's what holds them tight so they don't work loose. Once the nuts work loose, the nuts and the wheel must be replaced. That's because both friction surfaces will be deformed, no longer match, and will continue to work loose.

As long as I'm on a roll, I must also mention to never get any hint of petroleum product in the brake fluid. That includes engine oil, transmission fluid, power steering fluid, axle grease, and penetrating oil. One drop of any of those things will lead to rubber seals and other parts swelling. The fix for that is to replace every part that has rubber parts that contact the brake fluid. That is a very expensive repair. Professionals even wash their hands with soap and water to prevent getting fingerprint grease in the brake fluid.
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Tuesday, June 16th, 2015 AT 2:04 PM

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