When purchasing a new aftermarket rear disc brake kit, it is important to check your Mustang's rear axle housing to determine with certainty, the Ford housing flange and axle offset you have under your Mustang. When you following the steps in this article, you will help to ensure your rear disc brake kit install goes smoothly with the correct and compatible parts to your rear axle housing and axles. 

The various types of Ford housing flanges (IE Big Ford Or Small Ford) have different vertical and horizontal bolt hole spacing on the axle housing flanges. Because of this variance in hole spacing, it is necessary to take a measurement of both the vertical and horizontal bolt pattern spreads and in some cases the ID Measurement of the axle housing tube at the flange to ensure the correct mounting brackets are sent with your new aftermarket disc brake kit. Incorrect brackets will not allow you to bolt your new disc brake kit to your housing flanges and halt your rear disc brake install in a hurry. Also, a axle offset measurement should be taken to identify the offset distance of your axles from the axle housing flange. Again, this measurement is crucial to ensure you get the correct hub height on your rotors to ensure the rotor is centered in the caliper evenly. 

When performing these important measurements (Axle Flange bolt spacing, Tube ID and Axle Offset) it is important to use a precise measuring tool (Not a tape measure) which will allow you to measure to the 1000th of an inch of accuracy. As you will see when looking at the axle flange identification charts, these measurements are done to the 1000th of an inch. The axle offset measurement is done to the 100th of an inch. We STRONGLY recommend using a precise caliper measuring instrument to perform all measurements. Again, using a tape measure will provide inaccurate measurements which will increase the chance of not identifying your axle flange and/or axle offset correctly, resulting in incompatible parts shipped in your rear disc brake kit. 

Here is the link to the Axle Flange Identification Charts with Bolt Spread Measurements and diagram showing where to measure axle offset.