Head Bolt Failure (M156 6.2L AMG V8)
The M156 6.2L naturally aspirated V8 used in numerous AMG models from 2007-2015 has a well-documented head bolt issue. The original factory head bolts were manufactured from a material that is susceptible to stress corrosion cracking and hydrogen embrittlement. Over time, the bolts stretch and lose clamping force, allowing coolant to seep past the head gasket into the cylinders or externally. Mercedes released updated head bolts (part number revision) to address this issue.
Which models are at risk.
Warning signs to watch for.
Coolant loss with no visible external leak
White smoke from the exhaust, especially on cold start
Misfires on one or more cylinders
Coolant odor from the exhaust
Overheating or fluctuating coolant temperature
Coolant found in the engine oil (milky appearance on dipstick)
Why this happens.
Original head bolts manufactured with material susceptible to hydrogen embrittlement
Stress corrosion cracking of the bolt shanks over thermal cycles
Bolts lose clamping force, allowing the head gasket to leak
Extended exposure to coolant containing silicates can accelerate bolt degradation
High-performance driving increases thermal stress on the bolts
How we diagnose it.
- 01
Perform a coolant pressure test and monitor for pressure drop over 30 minutes
- 02
Check for combustion gases in the coolant using a block tester (chemical color change)
- 03
Monitor misfires with Star Diagnostic — persistent misfires on adjacent cylinders suggest head gasket breach
- 04
Inspect oil for coolant contamination (milky residue on filler cap or dipstick)
- 05
Borescope inspection of the cylinders for coolant residue on the piston tops
How we fix it.
Head bolt replacement with updated bolts (preventive, no gasket damage yet)
Addresses root cause before gasket damage; updated bolts resolve the material defect; preserves the engine
Still requires removal of both cylinder heads; significant labor; some shops quote even higher
Head gasket and head bolt replacement (gasket already leaking)
Complete repair with new gaskets, updated bolts, and resurfaced heads; thorough and reliable
More expensive due to additional parts; longer downtime; heads may need machine shop work
Engine replacement (severe damage from prolonged coolant intrusion)
Necessary if cylinder walls are scored or bearings are damaged from coolant in oil; fresh start
Highest cost; M156 engines are becoming scarce; long downtime
Keep it from happening.
Proactively replace head bolts with the updated Mercedes part number around 60,000-80,000 miles
Monitor coolant level weekly — any unexplained loss warrants immediate investigation
Use only MB-approved coolant (MB 325.0 specification) — avoid universal coolants
Do not ignore white exhaust smoke, even if intermittent
Consider a block test during every major service to catch early head gasket seepage
Common questions.
Were all M156 engines affected by the head bolt issue?
Not all, but a significant percentage. Early production M156 engines (2007-2011) are most commonly affected. Mercedes quietly updated the head bolt part number, and engines built after 2012 are less likely to have the issue. However, any M156 with original head bolts should be considered at risk.
Can I just replace the head gaskets without new head bolts?
No. Replacing only the head gaskets without addressing the defective head bolts will result in the same failure recurring. The updated head bolts are the critical component of the repair. Any reputable shop familiar with the M156 will insist on the updated bolts.
How do I know if my M156 already has the updated head bolts?
The only definitive way is to remove a head bolt and compare the part number. The updated bolts have a different part number and markings. If you purchased the vehicle used and have no records of a head bolt service, assume the original bolts are still installed.
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