The Best Gearbox for Your M104 Swap – What Actually Works
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So, you’ve got a Mercedes M104 sitting in your garage—maybe you’re going for a clean restomod, or maybe you’re just a lunatic cramming it into something no sane person would. Either way, the engine is sorted.
Now comes the real challenge: finding a gearbox that won’t make you hate yourself every time you shift.
Let’s break down the best options so you can get your inline-six masterpiece shifting like it should.
1️⃣ 6-Speed 716.649 – The Smartest Choice
✔️ Overdrive 6th gear = highway cruising without crying at the pump
✔️ Close-ratio gears = still fun when you smash the throttle
✔️ Handles M104 power levels comfortably
If you want a manual swap that feels OEM+, this is the move. Unlike older 5-speed doglegs, the 716.649 gives you the best of both worlds: tight, performance-oriented shifting without screaming at 4,000 RPM on the freeway. It’s not the cheapest option, but if you’re serious about making the swap enjoyable to drive, it’s hard to beat.
2️⃣ Getrag Dogleg (717.404) – Cool, But Not for Everyone
✔️ Motorsport pedigree = instant street cred
✔️ Great for N/A setups and aggressive driving
❌ No overdrive = constant high RPMs on long drives
❌ Hard to find & stupid expensive
The dogleg 5-speed is a classic. It shifts different—first gear is down and to the left, making quick 2-3-4 shifts super intuitive. If your M104 is staying naturally aspirated, it’s a cool, period-correct choice, especially in something like a 190E. But if you plan on actually driving long distances, the lack of overdrive will wear on you.
3️⃣ BMW Gearboxes – The Big Power Option
✔️ Stronger than most Mercedes options
✔️ Huge aftermarket support
❌ Requires adapter plates & custom work
If you’re going turbo M104, you’re probably pushing 400+ hp, and that’s when a BMW GS5 or ZF gearbox makes sense. They handle power better than the Mercedes units, and there are plenty of clutch and flywheel kits available. Just be prepared for some fab work to get it mounted up.
4️⃣ W203 C270 CDI 6-Speed – The Diesel Wildcard
✔️ Potentially budget-friendly
❌ Not really built for a high-revving M104
Some people experiment with the C270 CDI 6-speed from a W203, and sure, it can work. But let’s be real—this box was built for diesels, not screaming inline-sixes. If you’re on a budget and just need a manual, it’s an option, but don’t expect it to feel as sharp as the others.
Clutch & LSD – Don’t Cheap Out Here
Whatever gearbox you pick, don’t skimp on these:
🔹 Clutch: Sachs Performance or Kangaroo Kit for solid bite and longevity.
🔹 Differential: 2.82 LSD changes the way power hits the pavement—a must-have if you want to enjoy the swap properly.
Final Verdict – What’s Best?
🚀 Best All-Around: 6-speed 716.649 – balance of performance and daily drivability.
🏁 Best for N/A Purists: Dogleg 5-speed – old-school motorsport feel.
🔥 Best for Turbo Builds: BMW GS5/ZF gearbox – takes abuse and holds power.
💰 Budget-Friendly Option: C270 CDI 6-speed – works, but not ideal.
If you’re in the middle of an M104 swap and wondering what to go with, the 6-speed 716.649 is the move. It’s fun, smooth, and makes sense for real-world driving. But hey—if you’re building a race car, the dogleg or a BMW box might be the better call.