MN-12 to SN-95 Bolt Circle Change and Brake Upgrades

Douglas L. Fraleigh – August 2000

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Note: The information in this article is presented for informational purposes only. You are solely responsible for your application of this information and/or any actions you may take and/or any results. You are solely responsible for your safety and should ensure that your vehicle is properly supported while you work on it.

 

 

This information pertains to my 1995 Ford Thunderbird Supercoupe, equipped with 4 wheel disc brakes and ABS. If you have a MN-12 model prior to 1993, you will need to upgrade to 93+ front spindles and sway bar end links.

 

MN-12’s have a bolt pattern which consists of 5 bolts on a 4 ” circle. SN-95’s (Mustangs 94+) have a bolt pattern of 5 bolts on a 4 ” circle. As many more options are available, both for wheels and brakes, with the SN-95 bolt pattern, many MN-12 owners have changed, or desire to change, to the SN-95 pattern. I myself, also owning a 1998 Mustang Cobra, had the stock 17x8 SVT wheels available after adding 18” wheels to my Cobra. Additionally, as I had previously owned a 1990 SC, I was all too aware of the limitations of the undersized stock brake setup.

 

Previously, it had been possible to change the front bolt pattern using SN-95 parts, but the only solution to change the rear bolt pattern was redrilling the stock hubs. I’m happy to report that with the release of the 1999 SVT Mustang Cobra, rear hubs that interchange with the MN-12, are now available.

 

 

Front Hubs and Brakes


With the 93+ spindles, the front wheel bolt pattern change and brake upgrade is a true “bolt-on”. You need to be aware of wheel diameter, width, brake caliper clearance, and tire size. With the 17x8 wheels I used, which have 5.72” backspacing, a 255/45-17 tire is the maximum size possible. Other wheels with different offsets may have different maximums, and most run a front tire size of 245/45-17. See the parts list for the SN-95 items that are required. Ford recommends changing the hub retaining nut each time it is removed, and for the cost of the part, it’s cheap insurance.

 

1.)                1.)                Raise and support the front of the vehicle with both wheels off of the ground.

2.)                2.)                Disconnect the stock brake hose, remembering that brake fluid is corrosive.

3.)                3.)                Remove the stock caliper by removing the two 12mm bolts from the rear (15mm socket).

4.)                4.)                Remove the stock rotor. You may have one or two retaining clips on the wheel stud, which may be removed with pliers. If the rotor is rusted to the hub, liberal application of penetrating oil and the judicious use of a mallet is often required.

5.)                5.)                Remove and discard the retaining nut dust cover.

6.)                6.)                Remove and discard the hub retaining nut.

7.)                7.)                Remove the hub.

8.)                8.)                Clean the spindle and slide on the SN-95 hub.

9.)                9.)                Install the hub retaining nut, and torque to 250 lb-ft.

10.)            10.)            Install the retaining nut dust cover.

11.)            11.)            Repeat steps 2 through 10 on the other side of the vehicle.

 

At this point the bolt pattern change for the front has been completed. Any SN-95 brake kit may now be installed following the instructions supplied with the kit. I have used a 13” PBR Track kit from Baer, stock PBR calipers and 13” rotors from my 1998 Cobra, and now the Brembo 4-piston 13” brake kit from the 2000 Cobra R. The only issues you will need to be aware of are brake caliper clearance (17x8 1998 Cobra wheels clear all these systems, other year Mustang, or aftermarket, wheels may not) and brake lines. The brake calipers I’ve mentioned all utilize a different brake line bolt than the stock calipers, and the mounting point for the bolt requires modification to the stock brake lines, if you choose to reuse them. I had a custom set of stainless steel lines made.

 

 

Rear Hubs and Brakes


Thanks to the introduction of the IRS equipped 1999 SVT Mustang Cobra, rear hubs are now available which allow for a “bolt-on”, actually “press-in”, conversion of the rear bolt pattern. The 1999 Cobra rear hubs are 28 splines, just like the MN-12, and use the identical wheel bearings. The rear brakes, however, continue to be an area where ingenuity is required. You can choose to redrill the stock rotor for the new bolt pattern, and have the center bore machined to fit the new hubs. I chose instead to adapt the 11.65” rear rotors from a SN-95 Cobra. Please understand that this adaptation will require several modifications to the stock caliper, the rear knuckle, and the fabrication of a bracket to mount the caliper in the correct position relative to the larger rotors.

 

1.)                1.)                Raise ONE rear wheel off of the ground. This is necessary to allow removal of the hub retaining nut.

2.)                2.)                Loosen (do not remove) the hub retaining nut.

3.)                3.)                Lower the wheel.

4.)                4.)                Raise the other wheel off of the ground, and repeat step 2.

5.)                5.)                Raise the rear of the vehicle with both wheels off of the ground.

6.)                6.)                Disconnect the brake hose.

7.)                7.)                Detach and remove the parking brake cable.

8.)                8.)                Remove the caliper by removing the two 12mm bolts from the rear (15mm socket).

9.)                9.)                Remove the rotor.

10.)            10.)            Remove the hub retaining nut.

11.)            11.)            Remove the knuckle. Once the three bolts (one upper, two lower) are removed, you can use a 2 or 3 jaw wheel puller to remove the hub/knuckle assembly from the half-shaft.

12.)            12.)            Press out the old hub, and press in the new Cobra hub. Unless you own a hydraulic press, you’ll need to take the hub/knuckle assembly to a shop that is properly equipped. This is also an excellent time to inspect, and replace if necessary, the wheel bearings. I choose to replace them, and the bearing retaining clips. Again, I would view this preventative maintenance as cheap insurance.

 

Note: If you intend to upgrade to the 11.65” Cobra rear rotors, skip to the next section.

 

13.)            13.)            Loosely reinstall the knuckle.

14.)            14.)            Jack up the lower control arm until the weight of the vehicle is supported by the suspension, and torque the three bolts (140 lb-ft upper, 110 lb-ft lower).

15.)            15.)            Repeat steps 6 through 14 on the other side of the vehicle.

16.)            16.)            Lower one side of the vehicle and torque the hub retaining nut to 250 lb-ft.

17.)            17.)            Repeat step 16 on the other side of the vehicle.

18.)            18.)            You may now reinstall your modified stock brakes.

19.)            19.)            Bleed the brake system in accordance with the factory service manual.

 

Cobra Rear Rotors


The stock rear rotors are 10.125” in diameter, while the Cobra rear rotors are 11.65”. The stock rear rotors have an offset of 40mm, while the Cobra rear rotors have an offset of 45mm. Therefore, a bracket is required that relocates the stock caliper outward 19mm and towards the center of the vehicle 5mm. As the centerlines between the existing caliper mounting holes, and the required mounting holes, is only 19mm, some modifications to the knuckle (see picture 1) and caliper (see picture 2) are required. Also, as the arc of the Cobra rear rotors is greater than the arc of the stock rear rotors, the caliper must be modified to maintain 2-3mm clearance between the outer edge of the rotor and the inside of the caliper (see picture 2). All of these modifications can be performed with simple hand tools, and in fact, I used a variety of flat files. In addition, the stock caliper retaining bolts cannot be reused, and 12mm Allen head cap screws must be used instead.

 

picture 1 picture 2

 

 

I fabricated each bracket using 3/16 plate steel and 4 12mm x 1.75 nuts. After cutting the bracket to shape with a saber saw, I used a drum sander for the final shaping. I used the holes in the knuckle to locate two of the mounting holes and marked the location of the other two 19mm adjacent. After drilling the four holes on my drill press with a 15/32” bit, I chamfered the holes and used a flat file to ensure that the plate was flat. I then welded one pair of nuts to the rear of the plate, with the flats parallel to the centerline of the mounting holes. These nuts will be used to mount the bracket to the knuckle. I welded the other pair of nuts to the front of the bracket, again with the flats parallel to the centerline of the mounting holes, and drilled them out with the same 15/32” bit as they are used only as spacers. I was fortunate in that the height of the nuts I used corresponded to the required spacer height, your mileage may vary, and you may need to search for nuts of the correct height (10mm). The design of the bracket (see pictures 3 and 4) is such that none of the welds are structural.

 

picture 3 picture 4

 

 

I strongly suggest test fitting the knuckle/rotor/caliper combination on your workbench to ensure that all clearances are correct, prior to attempting to mount the combination on your car.

 

Once you are completely satisfied that all the clearances are correct, reinstall the knuckles and brakes.

 

1.)                1.)                Perform steps 13 through 17 in the previous section.

2.)                2.)                Attach the bracket to the knuckle from the front using 12mm Allen head cap screws and torque to 85 lb-ft.

3.)                3.)                Install the Cobra rotor.

4.)                4.)                Attach the caliper from the rear using 12mm Allen head cap screws and torque to 85 lb-ft.

5.)                5.)                Reattach the parking brake cable.

6.)                6.)                Reinstall the brake hose

7.)                7.)                Repeat Steps 2 through 6 on the other side of the vehicle.

8.)                8.)                Bleed the brake system in accordance with the factory service manual.

 

 

Parts List

 

Description

Part Number

Qty

Cost

Total

Front Hub

F6ZZ-1104-AA

2

$60.00

$120.00

Front Hub Retainer

F3LY-3B447-A

2

$5.50

$11.00

Dust Cover

F3DZ-1N135-A

2

$4.00

$8.00

Rear Hub

XR3Z-1109-AA

2

$100.00

$200.00

Rear Hub Retainer

FOSZ-4B477-A

2

$7.00

$14.00

Rear Wheel Bearing

E9DZ-1215-A

2

$48.00

$96.00

Bearing Retainer

N803955S

2

$1.50

$3.00

Cobra Rear Rotor **

F4ZZ-2C026-B

2

$75.00

$150.00

Allen Head Cap Screws

12mm x 1.75 x 30mm

8

$1.00

$8.00

Hex Nut

12mm x 1.75

8

$.75

$6.00

 

** As an alternative to the stock Cobra rear rotors, I used anodized and drilled rotors from Autospecialty Powerstop, which I obtained from Summit Racing (see picture 5).

 

Right Rear Rotor

AR-8146-R

1

$96.06

$96.06

Left Rear Rotor

AR-8146-L

1

$96.06

$96.06

 

picture 5