Mercedes M104 Rebuild, common faults & everything you need to know
Complete guide to the Mercedes straight-six - a chain-driven DOHC with variable intake cam timing. From head gasket and timing chain to forged internals, with factory-verified torque figures and workshop prices.
We have built and reconditioned Mercedes inline-sixes for decades. The M104 is the successor to the M103 - a straight-six with a cast iron block, aluminium cylinder head and four valves per cylinder. It is a robust, durable design with chain drive and variable cam timing on the intake camshaft. The iron bottom end is hard-wearing. What needs attention sits in the top end and the surrounding systems: the head gasket and deck flatness age, the timing chain guide rails wear, the crankcase ventilation and oil cover start to leak, and the wiring harness on early model years is destroyed by the heat. The M104 is also an interference engine, so a timing chain failure bends valves. We do everything from cylinder head reconditioning to complete engine reconditioning with our own machine shop and a 12-month warranty.
The M104 at a glance
Data for the M104 series (3.0 and 3.2). Torque figures read from the Mercedes factory manual (Mercedes WIS) and cross-verified.
| Variant | Displacement | Power | Torque | Compression |
|---|---|---|---|---|
| M104.98x (300E-24 / 300SL-24) | 2 960 cc | 220-231 hk | 280 Nm | 10.0:1 |
| M104.99x (320 / E320, 9.2:1) | 3 199 cc | 220 hk | 310 Nm | 9.2:1 |
| M104.99x (S320 / SL320) 3.2 | 3 199 cc | 231 hk | 315 Nm | 10.0:1 |
Note - Identity data: the power and torque figures in the table are published identity data (output), not tightening torques. They are taken from vehicle data and vary slightly with model year, market and injection system (the 3.2 variants are quoted at 310-315 Nm at 3 750 rpm). They are not read from a factory data sheet and should be double-checked against the exact chassis number.
Torque specs
Read from the Mercedes factory manual (Mercedes WIS, Engine 104, tightening torques) and cross-verified. Stretch bolts (head, main, connecting rod) are torque-plus-angle - always fit new ones. The M104 is a cast iron block with an aluminium head, and several fasteners exist in multiple bolt versions with different torques - always check which version is fitted.
Cylinder head & valvetrain
Engine block & bottom end
Timing chain & cam adjustment (VVT)
Oil pump, oil filter & drive belt
Note - Stretch bolts: the head, main bearing and connecting rod bolts are torque-plus-angle stretch bolts (TTY). Mercedes specifies new bolts, and Meksta always fits new bolts on disassembly. The reason: stress accumulation is invisible and the cost of new bolts is negligible against the risk of failure. The head bolts are tightened in sequence with a cold engine, and a torque-angle gauge (not a beam wrench) should be used for the angle stage.
Note - Bolt versions: The M104 came in several bolt versions during production. The connecting rod bolts have three versions and the vibration damper centre bolt has two (400 Nm grade 10.9 with four spring washers, or 370 Nm grade 8.8 with a conical washer from 09/93). Bolt and washer must not be mixed - check which version is fitted before you torque.
Note - Cam adjuster (VVT): The M104 has variable cam timing on the intake camshaft. The cam sprocket exists in several versions with different torques (18 / 22 / 20 Nm + 60 degrees), and the cam adjuster's cover, sensor and magnet nut have their own torques. We replace worn seals and check the adjuster's operation at every top-end rebuild.
Note - Intake and exhaust: The Mercedes factory manual gives no torque figure for the intake and exhaust manifold fasteners on the M104 (the procedure only says loosen/refit, replace gaskets). We use Mercedes' general standard bolt torque by thread size and therefore deliberately leave out a specific figure here rather than guess.
Factory tolerances (bearing clearance, deck height, piston clearance) for the M104 are documented separately and published once cross-verified. All measurements are checked against factory tolerances with our own machine shop.
Common faults on the M104
This is what we see most often when an M104 comes in. Symptoms, causes and how we fix it.
Head gasket & flatness deviation
High mileage • High- Symptoms
- Oil and coolant mixing, white smoke, overheating, coolant loss and bubbles in the expansion tank. Oil running down the side of the block.
- Cause
- The head gasket ages and starts to leak between oil galleries, coolant passages and cylinders. The aluminium head on an iron block can warp after overheating. Requires resurfacing and a new gasket with new stretch bolts.
Wiring harness deteriorates (early model years)
1992-1995 • High- Symptoms
- Brittle, cracked wire insulation in the engine bay, intermittent fault codes, rough running, starting problems and short circuits in the engine wiring harness.
- Cause
- Early M104 engines have a biodegradable wire insulation that dries out and cracks from the heat. Classic on 1992-1995 engines. Often requires replacing the engine wiring harness and sensor connectors.
Timing chain & guide rails wear
200 000+ km • High- Symptoms
- Rattle from the timing chain on cold start, chain length out of tolerance, camshaft position fault codes. In the worst case the chain jumps.
- Cause
- The plastic timing chain guide rails and tensioner age and wear. Neglected oil changes and high mileage accelerate the wear. Since the M104 is an interference engine, a chain failure bends valves - we inspect and replace the rails, tensioner and chain during a rebuild.
Oil leaks (valve cover & cam adjuster)
Common • Medium- Symptoms
- Oil down the block and on the manifold side, oil smell, drips under the engine. Oil in the spark plug wells.
- Cause
- Aged seals in the valve cover, around the cam adjuster and in the front timing chain cover. Common and not serious in itself, but oil on hot exhaust parts is a fire risk and should be fixed.
Cam adjuster (VVT) rattles
Common • Medium- Symptoms
- Rattle from the front on cold start, cam adjuster fault code, poorer response in the mid-range.
- Cause
- Internal wear and aged seals in the cam adjuster (intake cam) make the unit rattle and fail to hold the correct position. It is reconditioned or replaced, and the sprocket bolt version is checked on fitting.
Big-end and main bearing wear
High mileage • Critical- Symptoms
- A knocking noise from the lower engine under load, low oil pressure, metal shavings at oil changes.
- Cause
- Worn bearing shells after high mileage, oil starvation or neglected oil maintenance. Requires crankshaft grinding and new bearings.
M104 rebuild prices
Prices based on M104 specifications: 6-cyl inline, about 89 mm bore, 24 valves, 7 main bearings + 6 big-end bearings. We carry out cylinder boring, crankshaft grinding and cylinder head resurfacing with our own machine shop. All prices exclude VAT and parts.
Cylinder head reconditioning
Bottom end
Full rebuild
Performance
Cylinder machining (6-cyl, about 89 mm bore)
| Service | Price |
|---|---|
| Cylinder boring 6-cyl straight block (89 mm) | 4 977 kr |
| Plateau honing 6 cyl | 1 450 kr |
| Stress boring 6 cyl | 5 980 kr |
| O-ringing block 6 cyl (high-boost) | 3 600 kr |
| Deck resurfacing block 6 cyl | 2 700 kr |
Crankshaft work (7 mains + 6 big-ends, 12 counterweights)
| Service | Price |
|---|---|
| Crankshaft grinding (7 mains + 6 rods) | 5 000 kr |
| Polishing standard | 1 022 kr |
| Cleaning 6-cyl crankshaft | 920 kr |
| Straightening (large) | 1 400 kr |
| Camshaft journal grinding 14 (DOHC) | 4 312 kr |
| Remove/refit counterweights 12 | 3 066 kr |
Valve reconditioning (24 valves)
| Service | Price |
|---|---|
| Cutting 24 valve seats (standard) | 2 730 kr |
| Cutting 24 valve seats (large) | 4 158 kr |
| Valve guide replacement 24 (light alloy) | 3 822 kr |
| Manufacturing valve seat rings 24 | 4 620 kr |
| Bronze sleeves fitting 24 | 3 402 kr |
Resurfacing (I6, large cylinder head size)
| Service | Price |
|---|---|
| Cylinder head resurfacing 6 cyl (standard) | 1 800 kr |
| Cylinder head resurfacing 6 cyl (large) | 2 100 kr |
| Block resurfacing 6 cyl (standard) | 2 400 kr |
| Block resurfacing 6 cyl (large) | 2 700 kr |
| Resurfacing manifold 6 cyl | 3 800 kr |
Connecting rod work (6)
| Service | Price |
|---|---|
| Small-end bushing replacement 6 (Ø22 mm piston pin) | 3 360 kr |
| Manufacturing small-end bushings 6 | 4 347 kr |
| Length adjustment connecting rod 6 | 4 347 kr |
| Press-fit piston replacement 6 | 2 373 kr |
| Piston ring replacement 6 | 1 659 kr |
| Big-end resizing 6 | 3 528 kr |
| Inspection/straightening connecting rod 6 | 2 121 kr |
| Full reconditioning connecting rod 6 | 4 746 kr |
| Shot peening connecting rods 6 | 3 402 kr |
Pressure testing & balancing
| Service | Price |
|---|---|
| Cylinder head pressure testing 6 cyl (small) | 2 814 kr |
| Cylinder head pressure testing 6 cyl (large) | 4 200 kr |
| Pressure testing cylinder block | 1 400 kr /h |
| Rotating assembly balancing inline 6 cyl | 5 100 kr |
| Weight matching connecting rods 6 | 1 974 kr |
| Weight matching pistons 6 | 1 638 kr |
| Flywheel balancing | 2 100 kr |
| Balancing flywheel + clutch | 2 842 kr |
Indicative prices for the M104 configuration (6-cyl inline, about 89 mm bore, 24 valves, alloy head, cast iron block). Labour prices exclude VAT and parts (piston rings, bearings, gaskets, stretch bolts). Request a quote for an exact price after inspection.
Tuning the M104
The M104 is primarily a reliable naturally aspirated six. NA gives moderate gains, but the cast iron block takes boost well - the big power comes from a supercharger or turbo conversion on forged internals. AMG already built 3.6 versions on the same base.
- 1
Intake, exhaust & engine optimisation
A freer intake, an exhaust system and a well-developed cam profile give a moderate but noticeable NA gain with better response.
240-260 hk - 2
Supercharger / low-boost turbo
A supercharger kit or low-boost turbo with fuelling and management. The cast iron block handles low-boost charging well with the right margins and turbo work.
320-360 hk - 3
Forged internals + turbo
Forged pistons and con-rods, ARP bolts, balancing and a larger turbo. This opens the power window right up.
420-480 hk - 4
Full build
A complete bottom end, a ported cylinder head and a big turbo for track and drag. We build the whole chain in-house with performance engine building.
500+ hk
Questions & answers about the M104
What we hear most often from M104 owners.
What does it cost to rebuild a Mercedes M104?
A cylinder head reconditioning costs about 7 500 - 10 500 kr (labour), a complete engine rebuild about 28 500 - 43 500 kr. On top of that come parts (gasket, bearings, piston rings, stretch bolts), typically 8 000 - 20 000 kr depending on condition and variant. We give a fixed quote after inspection.
Is the Mercedes M104 an interference engine?
Yes. The M104 is an interference engine. If the timing chain jumps or snaps, the valves hit the pistons, which bends valves and often causes secondary damage. That is why the timing chain guide rails and tensioner are critical - we inspect and replace them during a rebuild.
What is the difference between the M103 and the M104?
The M104 is the successor to the M103. The M104 is a DOHC engine with 24 valves and four valves per cylinder, versus the M103's SOHC with 12 valves. The M104 also gained variable cam timing on the intake camshaft (cam adjuster) and more modern injection over the years (KE-Jetronic, then LH-SFI and HFM-SFI). Both are cast iron blocks with an aluminium head and chain drive.
How long does a rebuilt M104 last?
With a proper rebuild and regular maintenance: 250 000 - 350 000 km or more. The cast iron block is very hard-wearing. Keep an eye on the head gasket, timing chain, oil leaks and (on early cars) the wiring harness, and the M104 is one of Mercedes' most durable sixes.
Can you tune an M104?
Yes. Naturally aspirated, you get moderate gains via intake, exhaust and mapping. The big power comes from a supercharger or turbo: on stock bearings you often build 320-360 hp, and with forged internals 420-480 hp. The cast iron block is a rewarding base for boosted builds, and AMG already built 3.6 versions on the same base.
What is the wiring harness problem on the M104?
Early M104 engines (around 1992-1995) have a biodegradable wire insulation that dries out and cracks from the heat in the engine bay. The result is intermittent fault codes, rough running and starting problems. We replace the engine wiring harness and worn sensor connectors as part of a service or rebuild.
Should you rebuild or replace the engine?
For the M104: usually rebuild. A used engine has an unknown history and the same wear points (head gasket, timing chain, oil leaks). A rebuilt engine has known tolerances, new wear parts and a 12-month warranty. We ship worldwide.
See also: the diesel sibling Mercedes OM606 (straight-six diesel) and, for a comparison with BMW's contemporary straight-six, BMW M54. We service straight-sixes from both Mercedes and BMW.
Have an M104 that needs looking over?
Call us directly or send in a quote request. We reply within 24 hours.