I admit it, I’ve left a few washers behind after some car repairs. I often kid myself that it helps reduce the weight of the vehicle and improve fuel economy. But, in some cases the washer really does matter.
Washers help to distribute clamping forces, lock a bolt into place and place some tension on a bolt or screw. In some cases, it can even help to make sure the tip of the bolt does not go through the other side of a caliper bracket mounting tab on the knuckle.
Raybesto’s Bulletin BPI 12-07 details a problematic case of washer mis-installation on the 2002-2012 Nissan Altima that can cause severe discoloration and possible rotor hat separation on rear brakes because a washer not installed in the correct position.
On this application take NOTE on how the caliper bracket bolts are removed. One bolt has a washer and one bolt (top) without . The top bolt without the washer uses ABS sensor bracket as the washer.
If the bolts are reversed, the lower bolt will make contact with the inside of the rotor surface. It will create a groove just above the center of the rotor and will cause extreme overheating with the potential rotor hat separation.
I have been there a thousand times before, you remove the caliper guide pin bolts and the bracket bolts, It is not until you have to put it together that you realize the bolts are different lengths. If you are in this position, you can compare the bolts on the other side or look at the manual.
SOURCE: RAYBESTOS TECH TIPS