Piston And Connecting Rod

The iron coated cast aluminum alloy pistons are "domed" with valve reliefs in the crowns (1). The recess (2) directs the combustible mixture directly under the spark plug and prevents the combustion chamber from being divided into two parts.

Fig 1: Identifying Piston Crown And Recess

Courtesy of BMW OF NORTH AMERICA, INC.

Piston rings:

• First piston ring groove = square ring
• Second piston ring groove = taperface ring
• Third piston ring groove = two-part oil control ring
  NOTE: Due to the shaping of the piston crown and the wrist pin offset, the pistons are cylinder bank specific (crown index arrow always points to the front of the engine).

The forged steel connecting rod and cap is separated by the familiar "cracked" process. The large end is angled at 30° allowing sufficient articulation in a very compact space. The pistons are cooled by oil jets spraying under the exhaust side of the piston crown.

• Remove piston/connecting rod assembly from any cylinder of the bank (caution - use care around the oil spray jet).
• Remove crankshaft thrust bearing cap for visual inspection.
• Measure the piston diameter.
• Measure the cylinder bore that the piston was removed from.