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Precision Locking Differential Technology for the Subaru WRX STI
The OS Giken SB041-HA Super Lock LSD is a rear limited-slip differential engineered for the Subaru Impreza WRX STI GDB and GRB chassis, covering 2004 through 2021 model years on the EJ25 powertrain. OS Giken's patented pressure ring design allows for smooth, progressive locking behavior combined with a fully capable 100 percent lock when demanded by aggressive driving inputs. The SB041-HA uses a spline-through-side-gear architecture that accommodates up to twice as many friction plates as competing designs, translating to superior durability and consistent performance on street and track.
Key Features
- OS Giken patented pressure ring design for smooth progressive locking
- Spline-through-side-gear construction for maximum friction plate count
- 1.5-way ramp configuration: 25 degree acceleration, 38 degree deceleration
- Low preload with full 100 percent lock capability
- 20 total friction plates for exceptional torque capacity
- Raw-forged gears and high-grade steel alloy construction
- Compatible with EJ25-powered GDB and GRB STI chassis
Vehicle Fitment
| Year Range | Make | Model | Trim |
|---|---|---|---|
| 2004-2007 | Subaru | Impreza | WRX STI |
| 2008-2014 | Subaru | Impreza | WRX STI |
| 2008-2014 | Subaru | Impreza | WRX STI Limited |
| 2010-2013 | Subaru | Impreza | WRX STI Special Edition |
Specifications
| Detail | Value |
|---|---|
| Part Number | SB041-HA |
| Brand | OS Giken |
| Type | Limited Slip Differential |
| Location | Rear |
| LSD Style | 1.5-Way (25/38 Ramps) |
| Lock Timing | 336 |
| Total Plates | 20 |
| Preload | 10S |
| Fitment | Subaru WRX STI GDB/GRB EJ25 |
| Condition | New |
| UPC | N/A |
What's Included
- OS Giken SB041-HA Super Lock LSD unit (rear)
- Required internal hardware
- Installation documentation
About OS Giken
OS Giken is a Japanese precision manufacturer with a long history in motorsport-grade drivetrain components. Their LSD technology is used in Super GT, rally, and time attack competition globally, and is recognized for combining low preload street comfort with race-level locking performance in the same unit.
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Frequently Asked Questions
What is the SB041-HA?The OS Giken SB041-HA is a rear Super Lock limited-slip differential designed for the Subaru Impreza WRX STI GDB and GRB chassis with EJ25 engines. It uses OS Giken's patented pressure ring technology to deliver smooth, progressive locking with full 100 percent lock capability when required.
What does 1.5-way mean on the OS Giken SB041-HA LSD?A 1.5-way LSD uses different ramp angles for acceleration and deceleration. The SB041-HA uses 25 degree ramps on acceleration and 38 degree ramps on deceleration, providing aggressive traction on power while offering partial lockup during lift-off for predictable corner entry behavior.
Is the OS Giken SB041-HA suitable for daily street driving on a WRX STI?Yes. The SB041-HA's low preload setting makes it smooth enough for daily street use, while the progressive pressure ring engagement avoids the harsh binding found in some high-preload clutch-type LSDs. It is a practical upgrade for drivers who use their STI on both street and occasional track days.
Does the SB041-HA require any special fluid for the rear differential?OS Giken recommends using appropriate gear oil in the rear differential after installation. Consult the OS Giken installation documentation for the specific viscosity and capacity recommendations for the GDB/GRB rear differential housing.
Will the OS Giken SB041-HA fit both GDB and GRB Subaru STI chassis?Yes. The SB041-HA is designed to fit both the GDB (2004 to 2007) and GRB (2008 to 2014) Subaru Impreza WRX STI chassis that use the EJ25 engine with the rear differential housing common to both generations.
How does the OS Giken Super Lock LSD compare to a Cusco or Kaaz LSD for the STI?The OS Giken patented pressure ring design allows for more friction plates than most competitor designs, enabling higher torque capacity and longer service intervals. The progressive locking characteristic of the SB041-HA is frequently cited as more refined than equivalent Cusco or Kaaz units in the same application.
How long does the OS Giken SB041-HA last before requiring a rebuild?OS Giken LSDs are built with raw-forged gears and high-grade steel components rated for extended motorsport service intervals. Rebuild frequency depends on usage intensity, but track-focused drivers should inspect friction plate wear annually and replace plates per OS Giken service guidelines.
Can I install the SB041-HA myself or does it require professional installation?Rear differential replacement on the Subaru STI requires removing the rear differential and properly torquing ring gear and bearing preload. This work is best performed by a qualified Subaru specialist or performance shop familiar with GDB/GRB drivetrain assembly to ensure correct preload and backlash settings.
Installation Instructions
- Raise and support the vehicle safely on jack stands with the rear wheels off the ground.
- Drain the rear differential fluid and remove the driveshaft and axle shafts.
- Unbolt and remove the rear differential assembly from the subframe.
- Disassemble the differential housing and remove the factory LSD or open differential unit.
- Install the OS Giken SB041-HA Super Lock LSD into the housing, setting bearing preload and ring gear backlash per specification.
- Reassemble the differential housing with all fasteners torqued to Subaru specification.
- Reinstall the differential, driveshaft, and axle shafts.
- Refill with recommended gear oil and check for leaks before returning to service.
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