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Diffrential
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 |
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(Note: Click thumbnails to enlarge pictures)
Differential Paint Markings
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”.
The pictures below show additional paint marks found on the Boss 351 3.91 Traction Lok “N” Case.
Yellow dab on bottom of case.
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.
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
Traction Lock Unit
Detroit Locker
Detroit Locker
Detroit Locker
Differential
Axle Tube
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.
28 Spline Axle with 1 concave hole.
N Case Carrier
Daytona Support
Detailed “N” Cases
9″ Rear End Housing
Pinion Snubber
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
28 compared to 31
31 Spline
AXLES
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.
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
