The easiest way to tell is to try to put a socket on the lower bolts.  On the 8" shown here, the socket goes on straight without hitting the case.	On the 9" shown here, the socket won't fit this position or the bolt on the other side of center.  Note the pocket at the bolt location necessary because of the larger pinion support.

• 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

• 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

• 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 aftermarket gear sets.

• 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

• 1972 and earlier 31 spline axles have the ability to be shortened.
• 28 spline axles are tapered and cannot be shortened and re-splined.
• 1973 and later cars have a 5-on-5 bolt circle and the axles cannot be shortened.
• 1967-1973 Mustang axles can be identified by wheel flange:
  Oval hole = 28 splines.
  Two large holes and counter-sunk center = 31 splines.

• 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.

The 8” and 9” have an extra bearing in the front of the pinion to give it extra support. This third bearing reduces flexing of the pinion under hard acceleration. These rears also utilize the axle bearing to retain the axle, unlike C-clips used in some other rears. If an axle were to break, the C-clip retainer, located in the carrier, can not keep a broken axle in the housing. The drop-out carrier also makes maintenance easier.

Strength - There are four usable styles you can find in a junk yard. They range from light duty to extreme duty but in all cases the larger 31 spline axle versions are stronger than the 28 spline version of the same unit.

• The 67-73 Mustang type is considered light duty uses both axles.
• The 57-68 early passenger car and half-ton pickup rear end is considered a medium duty unit and came in both 28 and 31 spline axles.
• The 70-79 Ranchero/Torino are considered a high performance unit again being produced with both 28 and 31 spline axles.
• The 69-77 Galaxie/Lincoln and 73-86 half-ton pickup used both size axles and the 31 spline version should be considered the strongest available and can be used for extreme duty applications.

Conversion from 28 Spline to 31 Spline

To change the third member to 31 spline, the only parts needed are the differential side gears, they are available from Ford, about $20 apiece. The other part of the conversion is the carrier bearing size. There are two carrier bearing inside bore sizes, and two carrier bearing outside bore sizes. There are three possible combinations, small ID-small OD , large ID-small OD , and large ID-large OD. Factory 31 spline axles came with the last two bearing configurations, but not with the first. On 28 spline carriers, the axle bore on some of them are not large enough to allow the 31 spline axles to pass through. The carriers with the large ID bearings can be bored out to use the 31 spline axles. Not a precision job, just big enough for the axle to pass through, I use a cut off end from a 31 spline axle to check when it's large enough. I takes less than an hour to do both sides, and could be done at a vocational school by a novice. The small ID bearings won't have enough material left for reliability if bored out for 31 spline axles. Also if your housing has the small axle bearings, you will have to have axles custom made, as there are no factory axles with 31 splines,and small axle bearings. Both the large axle bearing, and the extra large axle bearing (some F150 trucks) had 31 spline factory applications. From about 1970 on, the big Fords, Mercury's, and Lincoln's has axles big enough to respline with 31 splines if they are shortened about 4" per side. Both 4 1/2" and 5" bolt patterns are available. If you want to redrill to a 4 3/4" Chevy bolt pattern, use truck axles, as there is more room for the new pattern without hitting the other holes. The drum pilot may have to be turned down, depending on the drum used. The Chevy drums of corresponding size to the ford brakes will fit if the pilot is turned down.