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BMW Differential Types Explained: Sizes, Generations and Housing Codes
BMW differentials are renowned for durability, efficiency, and performance potential. Whether you're eliminating one-wheel-spin, preparing a drift car, or rebuilding a worn unit, understanding BMW differential sizes and housing generations is essential.
Most enthusiasts recognize BMW diffs by size — 168mm, 188mm, and 210mm (215/220) — but fewer understand housing codes such as MM, K, L, LW, and AL and what they mean for compatibility and performance.
This guide explains:
• Differential sizes and identification
• Housing generations & mechanical changes
• Meaning of BMW diff codes
• Model fitment & timeline
• Performance implications
BMW Differential Sizes (Ring Gear Diameter)
BMW differentials are categorized by the outer diameter of the crown wheel (ring gear).
🔹 168mm — Small Case
Typical use: 4-cylinder and lower-torque engines
Common models: E30, E36, Z3, E46 Compact
Torque capacity: ~250–300 Nm
🔹 188mm — Medium Case
Most common BMW differential size
Used in: E30, E36, E46, E34, E39, E60, E90, Z3, Z4, X3, X5
Torque capacity: ~450–600 Nm (more with upgrades)
🔹 210 / 215 / 220mm— Large Case
High-performance & V8 applications
Used in: E34 M5, E39 M5, E36 M3 3.2, E60 M5, E63 M6, E32/E38 V8
Torque capacity: 700+ Nm
Identifying BMW Differential Size
MM Generation Identification
Early BMW differentials can be identified by counting the side cover bolts:
| Size | Side Cover Bolts |
|---|---|
| 168mm | 4 bolts |
| 188mm | 6 bolts |
| 210mm | 8 bolts |
These housings use threaded side caps and shim packs to set backlash and carrier position.
K, L, LW & AL Generations
Later designs eliminated side caps. Size identification is done by measuring the outer diameter of the axle oil seals:
| Size | Oil Seal OD |
|---|---|
| 168mm | 78 mm |
| 188mm | 90 mm |
| 215/220mm | 100 mm |
This method applies to all modern BMW differential generations.
BMW Differential Housing Generations
BMW refined differential housings over time to reduce friction, improve efficiency, and simplify assembly.
MM Type (Traditional Housing)
Used until ~1999
The MM housing represents BMW’s classic differential design.
Key Characteristics
✔ Threaded side caps with shim adjustment
✔ Carrier preload & backlash set via shims
✔ Extremely robust construction
✔ Higher internal friction compared to later designs
Important Note
✅ MM housings are the only generation where non-M BMW models were factory-equipped with clutch-type LSD units.
This makes them highly desirable among enthusiasts.
Common Applications
- E30
- E36
- E28 / E34
- E23 / E24
- E32
K Type (Compact Housing Generation)
Approx. 2000 – 2005
The K generation marked BMW’s first major modernization step.
Mechanical Changes
✔ Side caps eliminated
✔ Carrier retained by side C-clips
✔ Improved lubrication and structural rigidity
✔ Simplified assembly and service
Size Identification
Recognized by measuring axle seal diameter (78 / 90 / 100 mm).
Common Applications
- E46
- E39
- E38
- E53 X5
L Type (Low Friction Generation)
~2003 – September 2007
Introduced to reduce drivetrain losses and improve fuel efficiency.
Engineering Improvements
✔ Ball bearings replace tapered bearings in key positions
✔ Reduced rotational drag
✔ Lower operating temperatures
✔ Improved efficiency and smoother operation
Common Applications
- 1 Series: E81, E82, E87 (from introduction)
-
3 Series: E90, E91, E92, E93
- E85/E86 Z4
BMW introduced the L type in early 1-Series production and used it until September 2007.
LW Type (Low Friction + Welded Ring Gear)
September 2007 – Present
LW housings evolved from the L generation and remain in use today.
Key Improvements
✔ Ring gear welded to the differential carrier
✔ Increased rigidity & strength
✔ Improved torque transfer
✔ Eliminates ring gear bolt failures
⚠️ Requires machining when changing gear ratios.
Applications
- late E9X models
- late E8X 1-series
- late E60/E61
- E84 X1
- many later BMW platforms after September 2007
AL Type (Aluminium Housing)
BMW introduced aluminum housings primarily for weight reduction and thermal efficiency.
Advantages
✔ Significant weight savings
✔ Improved heat dissipation
✔ Reduced drivetrain mass
✔ Increased efficiency
Typical Applications
Most common in F-generation 5 Series and 7 Series, as well as other modern platforms.
Despite the aluminum case, internal gearing remains steel for durability.
Why BMW Introduced Low-Friction Differentials
BMW engineers redesigned differential housings to:
• improve fuel efficiency
• reduce drivetrain losses
• lower noise & vibration
• improve smoothness
While efficient, low-friction designs may reduce lock effectiveness, which is why performance drivers upgrade to clutch-type LSD solutions.
Quick Timeline Overview
| Generation | Years | Key Feature |
|---|---|---|
| MM | – 1999 | Side caps & shims, factory LSD option |
| K | 2000–2005 | C-clip carrier retention |
| L | 2003–09/2007 | Low-friction bearings |
| LW | 09/2007–present | Welded ring gear |
| AL | modern | Aluminum housing |
Why This Knowledge Matters
Understanding your BMW differential ensures:
✔ correct LSD upgrade selection
✔ compatibility with rebuild parts
✔ easier identification & servicing
✔ improved durability & performance
At RacingDiffs, we engineer solutions for all BMW differential generations, helping drivers unlock maximum traction on the street, track, or drift course.
Final Thoughts
BMW’s differential evolution reflects the balance between durability, efficiency, and performance. Knowing whether your unit is MM, K, L, LW, or AL allows you to select the right upgrades and fully exploit your drivetrain’s potential.
If you're unsure which differential your BMW uses, identification methods above will help — or contact RacingDiffs for expert support.
BMW Differential FAQ
How can I identify which BMW differential I have?
MM housings (pre-1999): count the side cover bolts:
- 4 bolts = 168mm
- 6 bolts = 188mm
- 8 bolts = 210mm
K, L, LW & AL housings (2000+): measure axle oil seal outer diameter (OD):
- 78 mm OD = 168mm
- 90 mm OD = 188mm
- 100 mm OD = 215/220mm
You can also confirm using the differential tag or vehicle VIN.
Which BMW differentials came with factory LSD?
Factory clutch-type LSD was available in:
- Most BMW M models (depending on market and model year)
- Selected non-M models with MM housing (pre-1999)
- Some performance packages and rare market variants
Many later BMW models use open differentials with electronic traction control instead of a mechanical LSD.
What does BMW “low friction differential” mean?
Low-friction (L and LW) housings were designed to reduce drivetrain losses by using ball bearings in key locations.
- Lower internal drag
- Improved efficiency and fuel economy
- Lower operating temperatures
- Smoother operation
What is the difference between L and LW differential housings?
L type: low-friction design with a bolted ring gear.
LW type: low-friction design with a welded ring gear for increased rigidity and strength.
LW started from September 2007 and is still used today.
Can a welded (LW) differential be rebuilt?
Yes, LW differentials can be rebuilt and upgraded.
Note: gear ratio changes typically require machining due to the welded ring gear, so professional tools and experience are recommended.
Why did BMW stop using factory LSD in non-M models?
BMW shifted toward electronic traction systems, efficiency improvements, and smoother drivability for mass-market models.
Enthusiasts still prefer mechanical LSDs for track use, drifting, and predictable traction.
Is an aluminum differential housing weaker than steel?
No. BMW aluminum housings (AL type) reduce weight and improve heat dissipation while remaining strong enough for factory torque levels.
The internal gears remain steel for durability.
Which BMW differential is strongest?
In general, strength increases with ring gear size:
- 210 / 215 / 220mm (large case)
- 188mm (medium case)
- 168mm (small case)
Real-world durability also depends on internal upgrades, setup, and driving style.
What is the benefit of upgrading to a limited slip differential?
A mechanical LSD helps eliminate one-wheel-spin and improves control:
- Better traction and acceleration
- More predictable corner exit
- Improved drift control and consistency
- Better performance in wet / low-grip conditions
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