M57 M15 HEAD BOLT CONVERSION

Technical guide to converting BMW M57 cylinder head bolt threads from M12 to M15 for high-boost applications.

WHY M15?

On high-power M57 builds, increased cylinder pressure can place significantly more load on the cylinder head clamping system. Converting the head bolt threads from M12 to M15 increases the available thread engagement and allows the use of larger bolts for improved cylinder head clamping.

This modification is mainly intended for heavily modified engines running high boost and high cylinder pressure. It is not necessary for every M57 build.

WHAT IS REQUIRED

  • M15x2 cylinder head bolts – Victor Reinz 14-32069-01
  • Correct drill size for the conversion
  • M15x2 thread tap
  • Enlarging the holes in the original washers to 16 mm
  • Drilling the cylinder head bolt holes to 16 mm
  • Enlarging the bolt holes in the head gasket to 16 mm (WEDM is recommended, an OEM head gasket is preferred)
  • Tightening the bolts to the specified torque of approximately 300–320 Nm

The original M12 threads in the engine block must be drilled to the correct tapping diameter before the new M15 thread is cut. Accurate alignment is critical. The drill and tap must remain square to the original bolt axis to avoid damaging the block or producing an incorrectly aligned thread.

All swarf and debris must be removed from the bolt holes before assembly. The threads should be checked carefully before the cylinder head is installed.

DISCLAIMER

This information is intended as a technical reference for experienced engine builders. Incorrect machining or assembly can permanently damage the engine block.

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