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Data on Ford 9 inch Diffs.

In addition to the information below, the following link provides a wealth of information on Ford 9 inch diffs, housings, carriers and axles. 

http://www.maliburacing.com/ford_9_inch/ford_nine_inch.htm

How to tell a 8 inch diff from a 9 inch

Look at the lower nuts that hold the carrier (pumpkin or diff head) in the housing. Picture using a socket with an extension to remove these. On an 8" you can remove all of the nuts this way. On a 9" you can't get to a couple of the lower ones, the pinion housing hangs down and blocks them. You have to use a ring spanner.  

Axle widths:   

Note: The lengths stated are for a complete axle from brake drum to brake drum. For 64-73 Mustangs subtract 5 inches to get the bare axle housing width.

65-66 Mustang 57.25 inches
67-70 Mustang 59.25 inches
71-73 Mustang 61.25 inches
77-81 Versailles 58.50 inches
74      Maverick 8" 56.50 inches
75      Mustang II 8" 57.00 inches
67-73 Mustang, Torino, Ranchero, US Fairlane 9" 59.25 inches to 61.25 inches
57-59 Ranchero and station wagon rears, 57.25 inches
66-77 Bronco 9", 58 inches
77-81 Granada/Versailles, 58 inches
67-71 Comet, Cougar, Mustang, US Fairlane, 59.25 inches
71-73 Mustang, 61.25 inches
64      US Falcon 58 inches
67      Cougar 60 inches
67      US Fairlane 63.50 inches (coil springs)
72      US Ford Van 3/4 ton, 68 inches, 
73-86 US Ford Van 3/4 ton 65.25 inches
57-59 Ranchero and station wagon 57.25 iches (narrowest 9" housing)
66-77 Bronco 58 inches but has 5-on-5 1/2 inch diameter bolt circle
67-73 Torinos, Rancheros, US Fairlanes 59.25 inches or 61.25 inches
67-71 Comets, Cougars, US Fairlanes 59.25 inches

Spring Perch Location. (Early Mustangs)

The location of the spring perches on the 1964-73 Mustangs is the same for all models, the extra width is all outboard of the spring perches. This means (for example) that a 70 mustang housing will fit into a 66 model, but will be 1 inch wider on each side. Fitting rear wheels with an extra 1 inch backspacing will then locate the tyres back in the same location, if required. (Note: be sure to check for differences in rear brake hose location and fittings between the housings).

Where to find a 9" rear axle

67-73 medium and big block Mustangs and Cougars
66-71 Fairlanes, Torinos, Montegos, Comets, and other Ford intermediates with big blocks
57-59 V8 Fords and Mercurys
77-81 Lincoln Versailles & Trucks  

Types of 9" axle housings 

67-73 Mustang/Cougar - light duty, thinnest housing material, small axle bearings, 28 and 31 splines
57-68 passenger car and 1/2 ton truck - medium duty, stronger than Mustang type, 28 and 31 splines
Ranchero/Torino - heavy duty thick wall housing, 3.25 inch diameter axle tubes with flat tops
69-77 Galaxies (coils), Lincolns (coils), and late pickups (leaf)- 3.25 inch diameter all the way to the backing plate, coil housings have upper control arm mount  

How to recognize 9" housing centres 

57 - no dimples, flat center band up the center of the rear cover, bottom drain plug.
58-59 - two dimples on back of housing, flat center band, some had drain holes
60-67 - two dimples, flat center band, oil level hole in back cover
63-77 Lincoln, LTD, Thunderbirds had 9.375 inch centres, housings were cut away at the gasket surface for ring gear clearance, one curved rib at the front top portion of differential, strong but no gears  

Types of 9" Axles 

28 spline axles cannot be shortened and resplined (they're tapered)
72 and earlier 31 spline axles have the ability to be shortened
73 and later 9" (cars) have a 5-on-5 bolt circle and the axles cannot be shortened
67-73 Mustang axles identified by wheel flange: oval hole - 28 splines, two large holes and counter sunk center - 31 splines.

Rear end ratios : 9 inch.

There were 18 offered on mustangs from 1965 till 1967. 9" conventional and 9" locking. The codes are as follows. A number designates a conventional while a letter indicates a locking rear end:

         1.....3.00:1      A.....3.00:1
         2.....2.83:1      C.....3.20:1
         3.....3.20:1      D.....3.25:1
         4.....3.25:1      E.....3.50:1
         5.....3.50:1      F.....2.80:1
         6.....2.80:1      G.....3.80:1
         7.....3.80:1      H.....3.89:1
         8.....3.89:1      I......4.11:1
         9.....4.11:1      K.....2.75:1
                               L......2.83:1
                               O.....2.79: 1  (67 year only)