ARB AMC Model 20 Air Locker Differential - RD187, 29-Spline
RD187
The ARB RD187 Air Locker Differential is a selectable locking differential designed for compatible AMC Model 20 and 8.875" axle applications using 29-spline axle shafts. Built for Jeep CJ restorations, trail rigs, Hummer H1 applications, and custom off-road builds, this air-operated locker provides fully locked traction on demand. With the flip of a dash-mounted switch, compressed air activates the internal locking mechanism, forcing both axle shafts to rotate together for improved traction through rocks, mud, snow, and challenging terrain.
When activated, its patented locking mechanism provides positive mechanical engagement for maximum available traction. The heavy-duty two-piece steel carrier and reinforced annular air seal are engineered for reliable operation under demanding drivetrain loads. The RD187 is designed for specific AMC Model 20 front IFS, rear IRS, and solid rear axle configurations, with gear ratio and carrier compatibility determined by the axle application.
Includes:
- (1) ARB RD187 AMC Model 20 Air Locker Differential Assembly
- (1) Dash-Mounted Activation Switch
- (1) Air Solenoid Valve
- Air Line
- Bulkhead Fitting
Important: A compatible ARB Air Compressor is required for operation and is sold separately. Verify the axle configuration, 29-spline shaft compatibility, and gear ratio carrier break before ordering.
Specifications
| Manufacturer | ARB |
|---|---|
| Part Number | RD187 |
| Product Type | Selectable Air Locking Differential |
| Axle Platform | AMC Model 20 / AMC 8.875" |
| Ring Gear Diameter | 8.875" |
| Axle Position | Front IFS / Rear IRS / Solid Rear - Application Dependent |
| Axle Spline Count | 29-Spline |
| Axle Shaft Diameter | 1.25" |
| Front IFS Gear Ratio | 2.73 & Down / 3.08 & Up - Verify Carrier Configuration |
| Rear IRS Gear Ratio | 2.73 & Down / 3.08 & Up - Verify Carrier Configuration |
| Solid Rear Gear Ratio | 2.73 & Numerically Lower - Verify Application |
| Carrier Compatibility | Application-Specific AMC Model 20 Carrier and Gear Ratio Configuration |
| Differential Operation | Selectable Open / Fully Locked |
| Engagement Type | Pneumatic / Compressed Air |
| Carrier Construction | Heavy-Duty Two-Piece Steel Design |
| Locking Mechanism | Patented Timed Locking Mechanism |
| Air Seal | Reinforced Annular Air Seal |
| Material | Steel |
| Installation Type | Differential Carrier Replacement |
| Primary Application | AMC Model 20 Traction Upgrades for Jeep CJ, Hummer H1 & Compatible Off-Road Axle Builds |
| Required Components | Compatible Air Compressor, Air Supply, 29-Spline Axle Shafts & Differential Installation Components |
| Known Incompatibilities | Incorrect Carrier Break, Axle Spline Count or Incompatible AMC Differential Configuration |
| Quantity | One Air Locker Differential Assembly |
Fitment
Designed for compatible AMC Model 20 and AMC 8.875" axle applications using 29-spline shafts. Vehicle fitment depends on the installed differential, carrier break, and axle configuration.
| Make | Model | Year Range | Axle Application |
|---|---|---|---|
| AM General | Hummer | 1992–2001 | AMC 20 Independent Differential - Verify Position |
| Hummer | H1 | 2002–2006 | AMC 20 Independent Differential - Verify Position |
| Jeep | CJ5 | 1976–1983 | AMC Model 20 Solid Rear Axle |
| Jeep | CJ7 | 1976–1986 | AMC Model 20 Solid Rear Axle |
| Jeep | J10 | 1980–1988 | AMC Model 20 Rear - Verify Axle |
| Jeep | Scrambler CJ8 | 1981–1985 | AMC Model 20 Solid Rear Axle |
| Jeep | Wagoneer | 1979–1986 | AMC Model 20 Rear - Verify Axle |
Fitment Notes:
- Designed for compatible AMC Model 20 / 8.875" differentials using 29-spline shafts.
- Front IFS and rear IRS applications require confirmation of the specific differential configuration.
- Solid rear axle applications require verification of the gear ratio carrier break.
- The supplied solid rear axle specifications identify 2.73-and-numerically-lower gear ratio compatibility.
- Not every Jeep CJ, J10, Wagoneer, or Hummer H1 uses the same axle and differential configuration.
- Verify ring gear mounting, carrier bearing dimensions, spline count, and current gear ratio before ordering.
- Requires a compatible compressed air supply for locker engagement.
- ARB air compressor sold separately.
- Professional differential installation and air line routing are recommended.
Builder Note: AMC Model 20 axle applications include both traditional solid rear axles and independent differential configurations. These platforms may use different carrier arrangements and gear ratio breaks. Always identify the installed differential and verify the required carrier configuration before selecting an Air Locker, particularly when working with older Jeep axles or modified off-road vehicles.
FAQ
What axles does the ARB RD187 Air Locker fit?
The supplied application information identifies compatible AMC Model 20 and AMC 8.875" differential configurations using 29-spline axle shafts. Exact fitment depends on axle position, carrier configuration, and gear ratio.
Will the ARB RD187 fit a Jeep CJ5 or CJ7?
The RD187 is listed for select Jeep CJ5 and CJ7 applications equipped with AMC Model 20 rear axles. Verify the installed differential and carrier gear ratio break before ordering.
Does the RD187 fit a Hummer H1?
The supplied fitment information includes select AM General Hummer and Hummer H1 applications. These vehicles use independent differential configurations, so the specific front or rear application and carrier compatibility must be confirmed.
What gear ratios are compatible with the ARB RD187?
The supplied specifications identify 2.73-and-down and 3.08-and-up carrier configurations for certain independent differential applications. Solid rear axle applications are listed for 2.73-and-down ratios. Confirm the specific carrier break with ARB before ordering.
What is a carrier break on an AMC Model 20 axle?
A carrier break separates gear ratio ranges that require different ring gear mounting positions. Selecting the correct carrier configuration ensures the ring gear can be installed and positioned correctly for proper gear engagement.
Does the ARB RD187 require an air compressor?
Yes. A compatible ARB air compressor and air supply system are required to activate the locking mechanism. The compressor is sold separately.
How does the ARB RD187 Air Locker work?
Compressed air activates an internal locking mechanism that connects both axle shafts, forcing the wheels to rotate together. When disengaged, the differential operates as a conventional open unit, allowing normal wheel-speed differentiation.
Can the RD187 be used in a daily-driven Jeep?
Yes. When disengaged, the Air Locker functions as an open differential for normal street driving. The locking function can be activated when additional traction is required off-road.
How is an ARB Air Locker different from a spool?
A spool permanently locks both axle shafts together, while an ARB Air Locker allows the driver to engage or disengage the locking mechanism. This provides improved maneuverability and normal differential operation when the locker is switched off.
Does the RD187 include the activation switch and air line?
Yes. The supplied kit information lists the Air Locker differential assembly, activation switch, solenoid valve, air line, and bulkhead fitting. The air compressor is sold separately.
Can I install the RD187 during a ring and pinion gear change?
Yes. Installing an Air Locker during a differential re-gear is practical because the original carrier is already being removed. Proper carrier bearing preload, backlash, and ring gear tooth contact pattern must be established during installation.
Is the RD187 compatible with every AMC Model 20 axle?
No. AMC Model 20 differentials were used in multiple axle configurations, including solid rear and independent applications. The correct spline count, carrier break, and differential dimensions must be verified before installation.
