AA-4000 Rear Axle

The Ford Parts Price Lists covered the AA rear axle assemblies under three category types. Category links follow.

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AA Rear Axle Overview

The gallery below shows an example of the three categories of rear axle types for identification purposes. During the AA production time period from December 1927 through February 1932, there were nine rear axle assemblies installed on the assembly lines as follows:


AA-4000-A – Worm 7.25:1 – AA’s without emergency brakes
AA-4000-B – Worm 5.17:1 – AA’s without emergency brakes
AA-4000-E – Worm 7.25:1 – AA’s with emergency brakes
AA-4000-F – Worm 5.17:1 – AA’s with emergency brakes
AA-4000-G – 1928/1929 Bevel 7.16:1
AA-4000-H – 1928/1929 Bevel 5.11:1
AA-4000-K – 1930/1931 Bevel 6.6:1
AA-4000-L – 1930/1931 Bevel 5.14:1
AA-4000-M – 1930/1931 Bevel – AA112 Standrive (drop center chassis)

AA Rear Axle Category Identification

Worm Type Rear Axle – 1927-1929

There were two worm type rear axle assemblies for trucks without emergency brakes (AA-4000-A with a 7.25:1 gear ratio and AA-4000-B with a 5.17:1 gear ratio). These two assemblies looked identical on the exterior. Internally, only the worm and worm gear were different. These two axles were used through 1928 and some in early 1929.

Ratio Stamping

Ratio Stamping

The initial AA’s had the AA-4000-B high speed rear axle with no external identification as to the gear ratio. Sometime in early 1928, the low speed AA-4000-A rear axle was made available. At that time the left center section of the axle housing was stamped with an “A” for the low speed axle and a “B” for the high speed axle. Refer to the image at the left which identifies the stamping location.

For trucks with emergency brakes the axle shafts and axle housings were longer (13/16″ for the axle shafts and 25/32″ for the axle housings). ?????????? is this true?????????The track (i.e. tread) was the same measurement as the prior A/B assemblies. AA-4000-E with a 7.25:1 gear ratio and AA-4000-F with a 5.17:1 gear ratio were available for the emergency brake equipped AA’s for 1929. A few late 1928 AA’s might have had emergency brakes as well. The difference between the E/F assemblies was the internal worm and worm gear only. Gear ratio identification was the same “A” and “B” stamping used for rear axles without emergency brakes. The July 24, 1929 Indianapolis service letters announced that the right hand axle housing with “HS” for high speed and “LS” for low speed axle was being used for gear ratio identification. There was no indication as to where on the right housing the stamping was located.

AA-4210-A/B Worm & Related Parts

The worm (i.e worm screw) drove the worm gear which in turn drove the rear axle shafts through the differential gears. Watch the video to see how this works.

Worm Screw & Parts

Worm Screw & Parts

Worm (or worm screw) – This part included integrated front and rear shafts. The worm was connected to the drive shaft by an internally splined coupler (part AA-4684). The coupler was installed onto the drive shaft with flat head clevis pin A-23861 (3/8″ x 2-3/32″) which was peened on both ends. The coupler was installed onto the splined shaft end of the worm with A-23860 plain pin (3/8″ x 2-12″), flat washer, and cotter. However, most installation observed have had two headless rivets peened on both ends (no pin). This makes it difficult to separate the drive shaft from the worm shaft for overhauls. The two different worms were:

AA-4210-A Worm – This was the worm for the low speed gear ratio and had four threads (i.e. four spiral teeth). It drove a 29 tooth worm gear which yielded a 7.25:1 gear ratio. Production of this worm screw and worm gear combination started in early 1928.

AA-4210-B Worm – This was the worm for the high speed gear ratio. At the start of production in 1927, part AA-4210 (with no suffix) was used. The “B” suffix was assigned when both low and high speed gear ratios were offered. This worm screw had six threads (i.e. six spiral teeth). It drove a 31 tooth worm gear which yielded a 5.17:1 gear ratio.

Roller Bearing & Sleeves – A combination of a roller bearing AA-4615 and a sleeve AA-4616 was installed on each end of the worm shaft. An upset on the sleeve fit into axle housing notches locked them in place. This can be seen in the video above. These were TT carry-over parts. On the front side of the front roller bearing, a retainer washer was used. Initially a TT carryover washer was used which was 1/16″ thick (part A-22437). It was replace with 1/8″ thick AA-4622 washer sometime in 1928. The back side of the rear roller bearing was held in place by the thrust bearing (discussed below).

Worm Thrust Bearing – There were several designs of the AA-4696 worm thrust bearing used during the production period.

  • ?????Figure 2 shows the three parts which made up the design 1 AA-4696 worm thrust bearing. The AA-4698 ball and retainer assembly was sandwiched between two AA-4697 collars to make up the assembly. These were TT carryover parts and were used from the start of production in 1927 through sometime in 1928 (possibly through October).
  • A new design thrust bearing was released in late 1928 under part AA-4696-A2 (supplied by Gurney). The 10 balls of this assembly were held between outer/inner rings (i.e. races) as a permanent assembly (see ?????figures 1 and 3). This bearing was changed to a 9 ball bearing in early 1929. A 10 ball AA-4696-A3 Federal supplied bearing was used interchangeably with the 9 ball Gurney bearing starting in mid 1929. Both the 9 ball Gurney and 10 ball Federal bearings were provided through serviced as AA-4696-A2 .
  • ????Figure 4 shows the hardware used to attach the thrust bearing to the rear shaft of the worm. The worm shaft had pin A-23767 set into the shaft to keep retainer AA-4695 from rotating. Cotter pin A-23566 was used to secure the A-21941 castle nut.

Worm Grease Retainer – At the front side of the front roller retainer washer there was a TT carryover grease retainer. This consisted of A-22455 felt washer followed by AA-4692 retainer cup (with the cup facing the rear and enclosing the felt washer). The cup fit into the flange at the rear of the torque tube. The felt washer and cup combination was replace with AA-4700 grease retainer assembly in late 1928 to early 1929 and is shown in ?????figure 1. This assembly also fit into the flange at the rear of the torque tube.

Other parts?????????????????

??????????????????????????? to be added – housings, axle shafts, spider gears, torque tube/drive shaft, radius rods

Bevel Gear Type Rear Axle 1928-1929

late 1928 through early 1929.

The July 24, 1929 Indianapolis service letters announced that the differential housing with “HS” for high speed and “LS” for low speed axle was being used for gear ratio identification. There was no indication as to where on the housing the stamping was located.

Bevel Gear Type Rear Axle 1930-1931

January 1930 through end of production


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