BMW M50 Rebuild, common faults & everything you need to know
Complete guide to BMW's modern inline-six - timing-chain-driven DOHC with VANOS. From timing chain rails to 600 hp turbo builds, with factory-verified torque specs and workshop prices.
We have built and rebuilt BMW straight-sixes for decades. The M50 is one of the most rewarding modern BMW engines to work on - the cast iron block is robust, the forged crankshaft runs in seven main bearings and the bottom end takes plenty of load. What needs attention is up top: the single-row timing chain's plastic rails go brittle over the years, and on the VANOS version (M50TU) the adjuster wears. The M50 is also an interference engine, so a timing chain failure bends valves. We handle everything from cylinder head reconditioning to complete engine reconditioning with our own machine shop and a 12-month warranty.
The M50 at a glance
Data for the M50 series. Torque specs read from BMW's factory torque manual (N600) and cross-verified.
| Variant | Power | Torque | Compression | Engine management |
|---|---|---|---|---|
| M50B20 | 150 hk | 190 Nm | 10.5:1 | Bosch M3.1 |
| M50B20TU VANOS | 150 hk | 190 Nm | 11.0:1 | Bosch M3.3.1 |
| M50B25 | 192 hk | 245 Nm | 10.0:1 | Bosch M3.1 |
| M50B25TU VANOS | 192 hk | 250 Nm | 10.5:1 | Bosch M3.3.1 |
Torque specs
Read from the BMW factory manual N600 (Tightening Torques / Anzugsdrehmomente) and cross-verified against BMW TIS and Bentley-based references. Stretch bolts (cylinder head, main, con-rod, flywheel) are angle-tightened - always fit new ones.
Cylinder head & valvetrain
Engine block & bottom end
Timing chain & VANOS
Intake & exhaust
Note - Stretch bolts: cylinder head, main, con-rod and flywheel bolts are angle-tightened stretch bolts. BMW 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 M10 with a Torx head (E12) and are tightened in sequence from the centre outwards.
Note - Timing chain: The M50 has a single-row roller chain with plastic guide rails that go brittle over the years. Since the M50 is an interference engine, a rail or tensioner failure bends valves. We always replace the guide rails and tensioner during a rebuild and check chain stretch.
NOTE - Harmonic damper: the centre bolt is tightened to a high torque and requires a holding tool. The hub is press-fitted onto the crankshaft nose.
Factory tolerances (bearing clearance, deck height, piston clearance) for the M50 are documented separately against the Bentley E36 manual and published once cross-verified. All measurements are taken against factory tolerances with our own machine shop.
Common faults on the M50
This is what we see most often when an M50 comes in. Symptoms, causes and how we fix it.
Timing chain rails break
Common • High- Symptoms
- Rattling noise from the front of the engine, especially on cold start. In the worst case a chain jump with bent valves.
- Cause
- The plastic guide rails go brittle and crack with age. The M50 is an interference engine, so a broken rail or tensioner can let the chain jump and the valves hit the pistons.
VANOS wear (M50TU)
VANOS models • Medium- Symptoms
- Rattle at idle and cold start, weaker pull low down, uneven running. Fault codes for camshaft timing.
- Cause
- Worn seals and stops in the VANOS adjuster on the intake camshaft. Common on high-mileage M50TU and early M52.
Cooling system failure
150 000+ km • High- Symptoms
- Overheating, coolant leaks, cracked expansion tank. Water pump with a plastic impeller that comes loose.
- Cause
- Aged plastic components in the cooling system (expansion tank, thermostat housing, pump impeller). Overheating often leads on to head gasket and flatness problems.
Head gasket & flatness deviation
After overheating • High- Symptoms
- White smoke, coolant loss, emulsion under the oil cap, pressure in the expansion tank.
- Cause
- Usually a result of overheating. The aluminium head warps and the gasket blows. During a rebuild the head is resurfaced and checked for cracks.
Oil leak
Common • Medium- Symptoms
- Oil on the manifold side, oil smell, drips under the engine. Oil in the spark plug wells.
- Cause
- Aged gaskets in the valve cover and oil filter housing. Common and not serious in itself, but oil on hot exhaust parts is a risk and should be fixed.
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.
M50 rebuild prices
Prices based on M50 specifications: 6 cyl inline, 84 mm bore, 24 valves, 7 main bearings + 6 rod 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, 84 mm bore)
| Service | Price |
|---|---|
| Cylinder boring 6 cyl inline block (84 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 M50 configuration (6 cyl inline, 84 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 M50
The M50 is a rewarding platform to build on. NA gives modest gains, but the cast iron block and forged crankshaft take plenty of boost - that is where the big power lives.
- 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.
170-200 hk - 2
Turbo conversion (low boost)
Turbo with an aftermarket manifold, fuel system and management. The stock bearings handle low-boost charging well with the right margins and turbo work.
300-400 hk - 3
Forged internals + larger turbo
Forged pistons and con-rods, ARP bolts, balancing and a larger turbo. This opens the power window right up.
450-550 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.
600-700+ hk
Questions & answers about the M50
What we hear most from M50 owners.
What does it cost to rebuild a BMW M50?
A cylinder head reconditioning costs around 7 500 - 10 500 SEK (labour), a complete engine reconditioning around 28 500 - 43 500 SEK. On top of that come parts (gasket, bearings, piston rings, stretch bolts), typically 8 000 - 20 000 SEK depending on condition and variant. We give a firm quote after inspection.
Is the BMW M50 an interference engine?
Yes. The M50 is an interference engine. If the timing chain jumps or breaks, the valves hit the pistons, causing bent valves and often secondary damage. That is why the timing chain guide rails and tensioner are critical - we always replace them during a rebuild.
What is the difference between the M50 and M50TU?
The M50TU (Technical Update, from September 1992) got single VANOS on the intake camshaft and updated engine management (Bosch Motronic M3.3.1 versus the earlier M3.1), plus slightly higher compression. The bottom end is essentially the same. VANOS gives better mid-range torque but can start to rattle on high-mileage examples.
How long does a rebuilt M50 last?
With a proper rebuild and regular maintenance: 250 000 - 350 000 km or more. The cast iron block and forged crankshaft are very durable. Keep an eye on the cooling system and timing chain, and the M50 is one of BMW's most reliable sixes.
Can you tune an M50?
Absolutely. NA gives modest gains, but the M50 responds very well to a turbo. On stock bearings you often build 300-400 hp, and with forged internals 600+ hp. Many use the M50 block as a base for turbo builds thanks to the robust cast iron block.
Is the VANOS rattle serious?
An early rattle is usually worn seals and stops in the VANOS unit and not acutely dangerous, but it gets worse over time and affects pull and running. It is fixed during a cylinder head reconditioning or as a separate VANOS overhaul. Do not confuse it with timing chain rattle, which is more serious.
Should you rebuild or replace the engine?
For the M50: usually rebuild. A used engine has unknown history and the same wear points (timing chain, cooling system, VANOS). A rebuilt engine has known tolerances, new wear parts and a 12-month warranty. We ship worldwide.
Got an M50 that needs looking at?
Call us directly or send in a quote request. We reply within 24 hours.