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Determining a rotor’s or drum’s serviceability based on available specifications.


Many manufacturers do not supply both machine to and discard thickness


No Machine to Specification Listed

Many vehicles no longer provide a machine to specification. This does not mean the rotor is not serviceable. Most of these rotors are allowed to be machined to the discard specification. This is not a practical procedure in a shop environment. This is especially true in the states where there are safety inspections performed. It is possible to perform a brake job and have the vehicle fail a state inspection only a couple of months after the brake job.

Figure 20.1 shows an example of this. In this example no machine to specification is given. Instead a “NS“ is listed. The “NS” is short for “Not supplied by manufacturer”.

Figure 20.1 No Machine To

In this case the machine to is calculated by added .015” to the discard specification provided. In the case of the 98 to 2000 Mountaineer the discard listed is .980”. Adding .015” to this would yield a machine to thickness of .995”.

No Discard Specification provided

In Figure 20.2, no discard specification is given. Instead a footnote number is listed. The footnote listed is number 2 which can be found at the end of the make section. Footnote 2 says:

“Discard when thickness is smaller than the minimum machining specification”

20_2_footnote example_rgb

This means the discard and machine to specification are the same number. This would not be practical for the customer. If the rotor were machined down to the machine to it would go beyond service in a very short period of time. Another brake inspection or a state inspection would result in the rotor being under specifications after just being serviced. The industry has adopted the position of creating a revised machine to specification by adding .015” to the number provided. In the case of the 2001-99 Cougar the machine to listed is .874”. Adding .015” to it will bring the revised machine to up to .889”. This is number you would use to determine whether the rotor can be machined or not. The .015” allows for normal rotor wear during the life of the friction material.

NOTE: Although it is best to use the aftermarket approach, it is acceptable to machine these rotors down to the machine to specification listed. The invoice should be marked “rotors at minimum machining/discard” to avoid future problems.

The same holds true for brake drums. There is no drum discard specification provided for the vehicles in the highlighted box. Instead, a footnote number 3 is listed and that footnote says:

“Discard when diameter is greater than the maximum machining specification”

This means the maximum machining diameter and discard diameter are the same for these vehicles. If the drum were machined up to the machine to it would go beyond service in a very short period of time. Another brake inspection or a state inspection would result in the drum being over specifications after just being serviced. The industry has adopted the position of creating a revised machine to specification by subtracting .015” from the number provided. In the case of the 2001-99 Cougar the machine to listed is 9.040”. Subtracting .015” from this value would result in a machine to specification of 9.025”. This is the number you would use to determine whether to machine the drum or not.

Bonus Tip:  ‘NS” does NOT mean “Not Serviceable” as some technicians have been led to believe. It means “Not Supplied”.

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