The determining factors in whether or not a rotor can be serviced are both dimensions and condition. Because of this it’s important to know when to service or replace the rotor in order to avoid incorrect information that leads to over or under selling.The dimensions determine whether it can be machined, replaced or in some cases left alone. Not understanding the meaning of the dimensions involved in making these decisions can lead to a rotor being machined when it shouldn’t be. Likewise, its possible a new rotor sells when the old rotor was still serviceable.
Understanding all of the critical dimensions of a rotor will allow accurate determination of the part condition and the type of recommendation that should be made. In the rest of this article, you will learn all of the important dimensions necessary.
Nominal thickness is the thickness of a new rotor. While this thickness is not normally used, it is listed in most specification books.
Machine to Thickness:
This is the thinnest point that a rotor can be machined to and still be put back into service. The “machine to thickness” is there to provide enough rotor material to last the life of one set of pads. Essentially, the idea is to make sure that the pad will wear through before the rotor does. To help you visualize, the average difference between nominal and machine to thickness is .050“ to .060“. The typical difference between machine to and discard (explained below) is .015“. See the image below:
One common myth, called throw away motors, says that late model vehicles have no meat on them and are thus not meant to be machined. This is not true. Again, the average rotor provides .050“ to .060“ with some giving as much as .100“ between nominal and machine to thickness. With the average machining taking off .010“ to .015“ there is usually room for at least on machining. The only true throwaways are marked like the one in Images below. The footnote (3) on this BMW denotes the rotor should be replaced not machined. True throwaway rotors are marked in this fashion.
Rotor Discard Thickness
The thickness at which a rotor should be replaced is called the discard thickness. One common understanding is that the discard thickness is a heat related dimension. For example, if a rotor is at or below discard thickness it cannot dissipate the heat generated, however, this is not correct. Discard thickness is the thinnest a rotor can wear to. If the brake pads wear to nothing the caliper piston won‘t fall out of the caliper housing. It has nothing to do with heat. See the image below.
Many vehicles with a rotor at or below discard stop without any issues. The only sure way to know that a part is below discard is to measure the it and compare the measurement to the specifications.
Parallesism (Disc Thickness Variation, DTV)
The two friction surfaces of a rotor are designed to be parallel to one another within a certain specification. The allowable tolerance is known as parallelism. It is also known as the rotor‘s disc thickness variation or DTV. See Below.
Other Rotor Factors
A common question presented about rotors involves the thickness of one friction disc to the other on vented rotors as shown in the image below. There is not a published specification for the allowable tolerance for the difference in the thickness of one disc to the other. Some rotors are actually designed intentionally with different thickness discs. It is generally accepted that small differences are acceptable and are either intentional or a result of manufacturing variations.
When machining, the thickness of one friction disc to the other should be taken into consideration. For example, if a rotor is gouged on one friction surface due to metal to metal contact with the brake pad but not the other, it could be possible to machine the gouge out and the rotor still be over machine to thickness. The problem would be that the gouged friction disc would be considerably thinner than the opposite disc. When faced with this situation common sense is the only guide. If the gouge is deep enough to result in having to remove a considerable amount of material it is probably best to replace the rotor.
Corrosion and rust can also have an impact on the rotor‘s ability to function correctly. The first image below shows clogged vent holes due to rust buildup. This part is thick enough to machine but the rust will prevent proper cooling. Likewise, the second image below has experience enough corrosion to actually weaken its structural integrity.
The rust has eaten away at the thickness of the cooling fins. This condition could result in the two halves of the part separating resulting in a catastrophic brake failure. Always perform a thorough visual inspection of the rotor to determine its condition and serviceability.
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