Whether a rotor can be serviced is based on its dimensions and condition. The decision to service or replace the rotor may be based on incorrect information leading to over or under selling.
The dimensions of a rotor 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 or a new rotor sold when the old rotor was still serviceable.
Understanding all of the critical dimensions of a rotor will allow accurate determination of the rotor’s condition and what type of recommendation should be made. Here is a description of the important dimensions of a rotor:
Nominal thickness is the thickness of the rotor when it is new. This thickness is not usually used on a daily basis but is listed in most specification books.
Machine to thickness:
This is the thinnest a rotor can be machined to and still be put back into service. The purpose of having a machine to thickness is to provide enough rotor material to last the life of one set of pads. The assumption is that if the pads are replaced and the rotors are over machine to they should not experience enough wear to allow them to go below discard thickness through the life of that set of pads. 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 Figure 19.1.
There is a common myth concerning what are usually called throwaway rotors. The myth states that most rotors on late model vehicles have no meat on them and are not meant to be machined or can‘t be. 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 always room for one machining and sometimes 2. The only true throwaway rotors are marked like the one in Figures 19.2 & 19.3. The footnote (3) on this BMW denotes the rotor should be replaced not machined. True throwaway rotors are marked in this fashion.
The discard thickness of a rotor is the thickness at which the rotor should be replaced. The common understanding of the definition of discard thickness is that it is a heat related dimension. It is generally understood that if a rotor is at or below discard thickness it cannot dissipate the heat generated. This is not correct. Discard thickness is the thinnest a rotor can wear to so that 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 Figure 19.4.
This definition makes sense if you apply it to what most technicians have experienced over the years. The average technician has serviced many vehicles with a rotor at or below discard and the vehicle stopped fine. The technician usually doesn‘t know the rotor is at or below discard until they measure the rotor and compare the measurement to the specifications.
Parallelism (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 Figure 19.5.
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 Figure 19.6. 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 a rotor, 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. Figure 19.7 shows a rotor that has clogged vent holes due to rust buildup. This rotor is thick enough to machine but the rust will prevent proper cooling. Likewise the rotor pictured in Figure 19.8 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 rotor separating resulting in a catastrophic brake failure. Always perform a thorough visual inspection of the rotor to determine its condition and serviceability.
Rotor thickness is not very exciting but crucial in doing a good brake job.