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General Information

For the 429 CJ Mustang, rear end ratios of 3.25 or 3.50 were available. Air conditioned cars were available with 3.25:1 only. The 429 SCJ Mustang came with either a 3.91 Traction Lok or a 4.11 Detroit Locker.

For the 429 CJ Cougar, rear end ratios were 3.25 or 3.50 only  (Traction Lok was optional). All air conditioned cars required the 3.25 ratio.

Link to Axle Detailing: 429 Mustang Concours Rear Axle Detailing

1971 Mustang/Cougar Rear Axle Codes
Vehicle/Engine Conventional Locking Ratio

429 CJ Mustang/Cougar

9 R 3.25:1*

429 CJ Mustang/Cougar

A S 3.50:1
429 SCJ Mustang N/A V 3.91:1
429 SCJ Mustang N/A Y 4.11:1
* Air Conditioning only available with 3.25:1

(Note: Click thumbnails to enlarge pictures)

Differential Paint Markings

diffPaint1a.jpg (28182 bytes)   diffPaint2.jpg (19562 bytes)   diffPaint3.jpg (25502 bytes)   diffPaint4.jpg (22872 bytes)

3.91 Traction Lok "N" Case Paint Markings

The pictures above are from an original '71 429 SCJ Mach 1 with a 3.91:1 Traction Lok. Note the following;

  • The yellow paint dab by the "N"

  • The blue color on the yoke and pinion support

  • A red paint daub on the bottom side of the yoke

  • The number "7" and "0" on the driver's side of the case

The pictures below are from a '71 Boss 351 also with the 3.91 Traction Lok. Note it also has the yellow paint dab above the "N".

 db3.jpg (44477 bytes)            db4.jpg (43250 bytes)

The pictures below show additional paint marks found on the Boss 351 3.91 Traction Lok "N" Case.

db1.jpg (71828 bytes)

Yellow dab on bottom of case.

db2a.jpg (35497 bytes)    db6.jpg (15772 bytes)

In the photo at left, note the pink paint mark on yoke. The photo at right shows this same pink mark  looking from case towards yoke.

db4.jpg (43250 bytes)              db5.jpg (20329 bytes)

There is an additional pink paint mark and white dot on the opposite side of yoke. The picture at left shows an overview and a closeup in the photo at right.



Differential/Axles and Related Components
tractionlok2a.jpg (33111 bytes)

Traction Lock Unit 

tractionlok2b.jpg (34948 bytes)


detlock3.jpg (23027 bytes)

Detroit Locker

detlock4.jpg (24396 bytes)

Detroit Locker

gear.jpg (35595 bytes)


case.jpg (35248 bytes)


axeltube.jpg (29401 bytes)

Axle Tube

31splinenew.jpg (51706 bytes)

31 Spline Axle with 3 holes. Note: The three hole rule is only to be used for small and mid size cars from 1971 and earlier.

28splinenew.jpg (38829 bytes)

28 Spline Axle with 1 concave hole.

Ncase5.jpg (84840 bytes)

N Case Carrier

Ncase6.jpg (77142 bytes)


Daytonasupport.jpg (44899 bytes)

Daytona Support

ncase.jpg (75683 bytes)

Detailed "N" Cases

ncase1.jpg (36233 bytes)


rearendfinished.jpg (32971 bytes)

9"  Rear End Housing

pinionsnubber1.jpg (18656 bytes)

Pinion Snubber

pinionsnubber2.jpg (18373 bytes)


yoke.jpg (11592 bytes)

Differential Yoke for N Cases with Daytona Pinion Support

Check out this excellent site on Ford Differentials to learn more: Ford 9" Differential Cases       

28 spline.jpg (14470 bytes)     28vs31spline.jpg (96720 bytes)        31 spline.jpg (42172 bytes)

28 Spline          28 compared to 31             31 Spline


The picture above shows a 28 spline axle. You can spot a 28 spline by the large concave hole on the end of each shaft. The other photo shows the more desirable 31 spline axle. These can be identified by the flat surface with 3 dimples on the end of each shaft. Note: The three hole rule is only to be used for small and mid size cars from 1971 and earlier.


detlock2.jpg (82535 bytes)       tractionlok2a.jpg (33111 bytes)       

Traction Lok

There are two different Traction Lok unit designs shown above. If you look closely, the earlier design version at left has four silver dollar size holes at the 3, 6, 9, and 12 o'clock positions. These four deep holes, machined into the  inside of the cast iron cover, act as stress risers and significantly weaken the unit. The later design shown above right, is stronger because it does not have these 4 holes.

Both of the Traction Lok designs shown above have a common weak point when used in very high horsepower applications. The cast iron cover can fracture around the perimeter at a thin point in the casting. The photo below shows the same unit with the broken piece in place (at left) and removed (at right). The Traction Lok is still OK for the street, but not high powered racing.


Johan Lindvall's 429 SCJ Mach 1