The short version
- The crank hub is the big one, and it can fail with no warning on tuned and standard cars alike.
- A tapping noise from the front of the engine when cold usually means camshaft alignment.
- Blue smoke on start-up or after idling points to valve stem oil seals.
- With turbos, it's more a question of when than if. And we'd change the oil far more often than BMW says.
The S55 is the 3.0 litre twin-turbo straight-six in the F80 M3 (2014 to 2018), the F82 and F83 M4 (2014 to 2020), and the M2 Competition and CS (2018 to 2021). It's a strong, hugely tuneable engine, but like every M engine it has a few known jobs. These are the ones we see most in the workshop, with what to look out for and what it costs to put right.
1. Crank hub
The S55's crank hub drives the timing chain but isn't positively locked to the crankshaft. Under a sharp torque spike it can turn on the crank, throwing the cam timing out, and in the worst case causing valve-to-piston contact. There's no warning, and we see it on manual and DCT cars, tuned and standard. We fit the Insane Performance 4-pin, one-piece hub, and we've fitted more than 100.
From £2,049. Read more: S55 crank hub explained.
2. Camshaft alignment
This is a separate fault from the crank hub; the two aren't related. The classic symptom is a knock or tap from the front of the engine when it's cold and warming up, which disappears once the engine is up to temperature. Because the noise goes away, it's easy to ignore. Left unchecked, it can damage the camshaft and the cam carrier, which is a much more expensive repair.
From £999.
3. Valvetronic actuator and eccentric shaft
Valvetronic varies how far the intake valves open. Either the actuator motor or the eccentric shaft it turns can fail, or both. The most common early sign is a "woodpecker" tapping noise when the car is first woken up. That usually points to a failing actuator motor. If it's left, further problems with the motor or eccentric shaft usually bring up a dash warning, poor running and limp mode.
From £849.
4. Valve stem oil seals
The sign is blue smoke, on a cold start and after the engine has been idling. A typical example: you sit at the lights for 30 seconds or more, pull away, and the car leaves a cloud of blue smoke. It clears, and the car's fine while it's moving, until the next cold start or long idle. The seals harden with age and heat and let a little oil down the valve stems, where it burns.
We've seen it on low and high mileage cars alike, although the higher the mileage, the higher the risk.
From £1,699.
5. Turbos
With S55 ownership, turbos are more a question of when than if. On average they last at least 30,000 to 40,000 miles, sometimes more. We offer OEM refurbished units as well as hybrid upgrades where they suit the car, and we've used DRW, OEM and hybrid turbos for many years with great results.
From £1,999. If you're upgrading turbos, we'd encourage doing the crank hub first.
6. Rod bearings
Not a known weak point on the S55 the way they are on the S65 and S85. The factory shells last very well, and we'd recommend them from around 100,000 miles, or sooner on heavily tuned or tracked cars. On an S55 they're usually done alongside a crank hub upgrade.
Price on application. Read more: S55 rod bearings: are they worth doing?
Keeping an S55 healthy
BMW's service intervals are fairly extreme. We'd always recommend a shorter oil service interval of around 5,000 to 7,000 miles, depending on how the car is used. Fresh oil is cheap compared with almost every job on this list. Our major and minor services are from £299.
