BMW N55 Rebuild, common faults & everything you need to know
Complete guide to BMW's first inline-six with a twin-scroll turbo - timing-chain-driven DOHC with Valvetronic, double VANOS and direct injection. From wastegate rattle and charge pipes to 600 hp single-turbo builds, with factory-verified torque specs and workshop prices.
We have built and rebuilt BMW straight-sixes for decades. The N55 is the successor to the N54 and BMW's first inline-six with a single twin-scroll turbo - an aluminium block with an aluminium bedplate, double VANOS, Valvetronic and high-pressure direct injection with solenoid injectors. The bottom end is shared with the N54 and takes plenty of boost, which makes the N55 a rewarding tuning engine. What needs attention is around charging, cooling and seals: the wastegate arms in the turbo start to rattle, the plastic charge pipe cracks, the oil filter housing gasket leaks, and the electric water pump and VANOS solenoids wear. The N55 is also an interference engine, so a timing chain failure bends valves. We handle everything from cylinder head reconditioning and turbo reconditioning to complete engine reconditioning with our own machine shop and a 12-month warranty.
The N55 at a glance
Data for the N55B30 series. Torque specs read from BMW's factory manual (BMW TIS) and cross-verified.
| Variant | Power | Torque | Compression | Vehicles |
|---|---|---|---|---|
| N55B30M0 Twin-scroll | 306 hk | 400 Nm | 10.2:1 | 335i, 135i, 535i, X3/X5 35i, 640i, 740i |
| N55B30O0 M PerformanceTwin-scroll | 320 hk | 450 Nm | 10.2:1 | M135i, M235i, 135is |
| N55B30T0 Twin-scroll | 370 hk | 465 Nm (500 Nm overboost) | 10.2:1 | M2 (F87), X3/X4 M40i |
Torque specs
Read from BMW's factory manual (BMW TIS F30 335i, N55, tightening torques) and cross-verified against two independent readings of the same TIS pages. Stretch bolts (cylinder head, main, bedplate, con-rod) are angle-tightened - always fit new ones. The N55 has an aluminium block with an aluminium bedplate and M11 cylinder head bolts with two angle stages.
Cylinder head & valvetrain
Engine block & bottom end
Flywheel / drive plate
Timing chain & valvetrain
Charging, fuel, intake & exhaust
Note - Stretch bolts: cylinder head, main, bedplate and con-rod bolts are angle-tightened stretch bolts (TTY). 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 M11 and are tightened in sequence with two angle stages after the pre-torque - easy to lose count, so we colour-mark the front face.
Note - Flywheel differs by gearbox: both the automatic gearbox's drive plate/flexplate (M12x19) and the manual gearbox's dual-mass flywheel (DMF, M12x25) are tightened to the same value, 60 Nm plus 45 degrees, but the bolt type and length differ. Always check which gearbox the car has and use the correct bolt before tightening. The bolts are always replaced.
Note - Aluminium block with bedplate: The N55 has an aluminium block with a bolted-on aluminium bedplate (lower crankcase). The blue-marked aluminium bolts in the bedplate have their own torque figures, separate from the steel-threaded main bearing bolts - never mix them up. The high-output variant N55B30T0 (M2, X3/X4 M40i) also has cast iron liners and a forged crankshaft from the S55.
Note - Integrated exhaust manifold & turbo: The N55 has no separate cast exhaust manifold. The exhaust manifold is integrated into the cylinder head and the single twin-scroll turbo bolts directly to the head. The flange nut is tightened to a low torque (13 Nm) with a new nut - never tighten harder. The torque figures in the variant table (400, 450, 465 and 500 Nm) are the engine's power output, not tightening torques.
Factory tolerances (bearing clearance, deck height, piston clearance) for the N55 are documented separately and published once cross-verified. All measurements are taken against factory tolerances with our own machine shop.
Common faults on the N55
This is what we see most often when an N55 comes in. The bottom end is strong - most faults sit around charging, cooling and seals.
Wastegate rattle in the turbo
Common • High- Symptoms
- A rattling or clattering noise at idle and low revs, worst when the engine is hot. Sounds like loose metal or a broken heat shield.
- Cause
- Wear in the wastegate arm's bearing in the single turbo lets the flap play. Very common on high-mileage N55s and one of the engine's best-known weaknesses. Over time it can give poorer boost control. Fixed with a turbo reconditioning or an upgraded turbo.
Plastic charge pipe cracks
Common • High- Symptoms
- Sudden power loss, a big drop in boost, an underboost fault code. Often under hard throttle.
- Cause
- The OEM plastic charge pipe (between the turbo and throttle) goes brittle and cracks, especially if boost has been raised. The standard fix is an aluminium charge pipe. A common and cheap upgrade when tuning.
Oil leaks (oil filter housing & valve cover)
Common • Medium- Symptoms
- Oil on the underside, oil smell, drips, oil in the spark plug wells. Sometimes oil finding its way into the cooling system via the oil cooler.
- Cause
- Aged gaskets in the oil filter housing (with its integrated oil cooler) and in the valve cover. Common and not serious in itself, but oil on hot exhaust and turbo parts is a fire risk and should be fixed.
Water pump & VANOS solenoids
150 000+ km • Medium- Symptoms
- Overheating or a warning light (electric water pump dying), plus rattle, uneven running and camshaft timing fault codes when the VANOS solenoids clog up.
- Cause
- The electric water pump has a limited lifespan and often dies without warning. The VANOS solenoids clog with oil deposits. Both are common maintenance points on the N55. We clean/replace the solenoids and check the VANOS function.
Carbon build-up on intake valves
100 000+ km • Medium- Symptoms
- Gradually poorer throttle response, rough idle, misfires low down and increased fuel consumption.
- Cause
- Like all direct-injection (GDI) engines, the N55 gets carbon build-up on the intake valves because the fuel no longer washes over them. The deposits build up over time. Fixed by walnut blasting the intake ports.
Big-end and main bearing wear
High mileage • Critical- Symptoms
- Knocking noise from the lower engine under load, low oil pressure, metal shavings at oil changes. Made worse by high boost.
- Cause
- Worn bearing shells after high mileage, oil starvation or hard boost. Requires crankshaft grinding and new bearings. On tuned N55s this is one of the most important things to keep an eye on.
N55 rebuild prices
Prices based on N55 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 N55 configuration (6 cyl inline, 84 mm bore, 24 valves, alloy head, aluminium block with aluminium bedplate). Labour prices exclude VAT and parts (piston rings, bearings, gaskets, stretch bolts). Turbo and fuel system work is quoted separately. Request a quote for an exact price after inspection.
Tuning the N55
The N55 responds very well to mapping. An ECU remap and higher boost alone give big gains, and the bottom end (shared with the N54) takes a lot before it needs building - the really big power comes from a single-turbo conversion on forged internals.
- 1
Map, charge pipe & downpipe
A well-developed ECU remapping, an aluminium charge pipe and a freer downpipe make better use of the stock turbo. A big gain for little money on a completely stock bottom end.
380-420 hk - 2
Stock turbo max + fuel
Higher boost, an intercooler, a fuel pump upgrade and port injection or E85. The stock turbo and bottom end handle this well with the right margins and a turbo reconditioning if they have covered a lot of miles.
450-480 hk - 3
Single turbo + forged internals
Forged pistons and con-rods, ARP bolts, balancing and a big single turbo. This opens the power window right up.
550-600 hk - 4
Full build
Complete bottom end, ported head and a big single turbo for track and drag. We build the whole chain in-house with performance engine building.
600+ hk
Questions & answers about the N55
What we hear most from N55 owners.
What does it cost to rebuild a BMW N55?
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) and any turbo and fuel system work, typically 10 000 - 25 000 SEK depending on condition. We give a firm quote after inspection.
Is the BMW N55 an interference engine?
Yes. The N55 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 check and replace them during a rebuild.
What is the difference between the N54 and N55?
The N54 has two turbos (twin-turbo) and piezo direct injection. Its successor, the N55, switched to a single twin-scroll turbo, solenoid injectors and Valvetronic. The N54 has higher power potential per pound and is the classic tuning engine, while the N55 is simpler and usually cheaper to run. The bottom end is very similar. See our page on BMW N54 for the twin-turbo predecessor.
How long does a rebuilt N55 last?
With a proper rebuild and regular maintenance: 250 000 - 350 000 km or more at reasonable power levels. The bottom end is durable. Keep an eye on the wastegate, charge pipe, water pump and VANOS solenoids - and the rod/main bearings on tuned examples - and the N55 is a durable engine.
Can you tune an N55?
Yes, and it is one of the engine's biggest strengths. A map and a charge pipe alone give big gains. On the stock bottom end and stock turbo you often build 380-480 hp, and with forged internals and a big single turbo 550-600+ hp. At the high power levels it is the rod and main bearings plus the fuel system you need to build.
What is the wastegate rattle on the N55?
The wastegate arm's bearing in the N55's single turbo wears, so the flap starts to play and rattle at idle and low revs, worst when the engine is hot. It is one of the engine's best-known weaknesses. In time it can give poorer boost control. We fix it with a turbo reconditioning or an upgraded turbo.
Should you rebuild or replace the engine?
For the N55: usually rebuild. A used engine has unknown history and the same wear points (wastegate, turbo, bearings). A rebuilt engine has known tolerances, new wear parts and a 12-month warranty. We ship worldwide.
See also: the N55's twin-turbo predecessor BMW N54 and the naturally aspirated relatives BMW M54 and BMW M50. We service BMW's inline-sixes from the M20 to the N55, and carry out turbo reconditioning on the N55's twin-scroll turbo.
Got an N55 that needs looking at?
Call us directly or send in a quote request. We reply within 24 hours.