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I have a chance to get a ford 9 inch rear with 3.55 to 1, disc brakes and posi...
what is the width I need to install said rear in my 51 AD? Is there anything else I should look at to make sure I am buying the right one?

Parking brakes? Ideally it would be about 62 inches wide across the wheel mounting surfaces.

so basically 62 inches from rotor to rotor?

Where are you measuring on the rotor? Usually the surface the pads ride on are offset from the surface the wheels mount on.

Its that surface the wheel mounts on that determines where the wheels and tires will sit relative to the inner and outer fender surfaces.

Wheel mounting surface to wheel mounting surface. As Steve points out, setting up the parking brakes so they function on some diffs is critical to get the rear pads properly adjusted and the brakes fully bled. I ran into this on an Aussy Borg Warner posi/disc. You had to actuate the emergency brake cables to spin an internal ratchet in the caliper body that adjusted the pads. The cables were pricey and the rear discs wouldn't bleed or function properly without the emergency brake system operational. You might want to check a FoMoCo manual to determine what hoops you have to jump through to get the rear discs operational. Hope this helps.

Dave

I will look into it, all of this speculation stems from the 327 camaro engine I just bought...
it's a builder... but it's sweet

mike1951

I don't know what your budget is, but look into the IMCA modified ford 9 inch. It is a really sweet setup. You can spec the width, axle splines, and choose gear ratios and posi. Best of all you are starting with a new housing, axles, and bearings. Another option for a park brake is to use a pinion mounted brake.

John

A pinion mounted E brake is a good alternative however remember if jacked up they are not functional. Be safe.
I did some research on the Ford 9" and Ford 8" rears and what I saw was nothing that would fit the AD without cutting axles. Currie Enterprises offers several options (link below) quite pricey but dependable stuff.

http://www.currieenterprises.com/cestore/all9inford.aspx

I used a ford 9" from a 78 Lincoln Mark V. It fit perfectly with no width modifications in my '51 Chevy half ton.

I am not satisfied with the emergency brake mechanism on the stock ford calipers. In order for the brakes to keep adjustment so the parking brake will work, the brake caliper pistons have to rotate. I disassembled, cleaned, and reassembled the calipers with new rubber components. The pistons did not want to rotate freely. Now that I have run my truck for over a year, I might see if I can get the pistons to rotate now. Another alternative might be to purchase rebuilt calipers.

I forgot to mention the IMCA 9 inches come with disc brakes... you need to get the gm callipers, and you can spec out any lug pattern. I spec'd mine to use 12 inch 1.25 thick rotors, and 6 on 5.5 lug pattern. You can get them almost any length. I spec'd mine to be 61 inches wheel mount to wheel mount to center 7 inch rims under my 1940 1/2 ton fenders.

I mentioned the pinion brake in my earlier post to provide an option to the parking brake callipers. It provides a completely seperate system for stopping a vehicle.

Love pinion brakes. They function exactly the same as the driveshaft drum brake that was on all big trucks for 60 years or so and on open driveshaft cars like chryslers. Doesn't have all the cables, arms, equalizers, garbage inside the rear drum or rotor, and best of all they have their braking power multiplied by the rear end ratio. A 100#'s applied at the pinion with a 3.50 rear is 350#'s at the tires.

If you will be running a 327 the 9" Ford is a little "over kill". Some of the early Chevys (nova, camaro,firebird, etc.) will work for those horses and most will be a fit width wise for your AD.