This is what I have found so far when looking into this for myself.
Axle Widths:
65-66 Mustang 57.25 inches
67-70 Mustang 59.25 inches
71-73 Mustang 61.25 inches
77-81 Versailles 58.50 inches
67-73 Mustang, Torino, Ranchero, 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, Fairlane, 59.25 inches
71-73 Mustang, 61.25 inches
64 Falcon 58 inches
67 Cougar 60 inches
67 Fairlane 63.50 inches (coil springs)
72 Ford Van 3/4 ton 68 inches
73-86 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, Fairlanes 59.25 inches or 61.25 inches
67-71 Comets, Cougars, Fairlanes 59.25 inches
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Where To Find:
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
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Type Of 9" 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
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How To Recognize 9" Housing Centers:
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 centers, 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
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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
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More Facts:
5 on 5 lug pattern is a truck pattern
A 9" complete rear axle is approximately 35 pounds heavier than an 8.8 rear axle with approximately the same components.
It is common to find a 9" in a old Falcon or Comet that has had a HIPO SMALL block with 31 splines and a locker.
If the case has two verticle RIBS ,from top to middle of case it is a good IRON type IF in the very center of this case there is the letter "N" than this is a NODULAR CASE (GREAT) this is the strongest factory case made BY FORD!!!
The ranger truck works good with a 9" from a 64 to 71 Falcon, Comet, some 65 to 69 Mustangs or 64 to 67 Ranchero V8 cars.
