Information on diagnostics

Piston pin fractures

What could happen if you use starting aids such as starting spray too often? Why is the piston pin broken? Was it installed incorrectly? Or is the tuning to blame for the damage? You can find the most common causes for piston pins breaking in this article.

General information about piston pin fractures

Piston pin fractures can occur as a result of overstressing caused by abnormal combustion or by foreign bodies in the combustion chamber. Excessive or inappropriate use of starting aids (e.g. starting spray) should be viewed in the same way as the effects of extreme abnormal combustion.

The piston pin is deformed into an oval shape by the pressure exerted on the piston from the combustion gases. Under excessive loads a longitudinal crack can form at the ends of the piston pin, with its starting point either at the outer or inner diameter of the pin. The crack then spreads as a fatigue fracture towards the centre of the piston pin. In the area between the piston pin bore and the connecting rod eye that is subjected to the greatest shear stresses and bending stress, the crack then changes direction and becomes a lateral crack. This then ultimately causes the piston pin to break right through. In addition to the damage described here, fractures can also arise as a result of some other kind of damage.

Fractured piston pin

Description of the damage

  • Lateral fracture on the piston pin (Fig. 1) at the transition between the connecting rod and the piston pin boss.
  • Shorter fragment split along its length.
  • Fracture surfaces display the characteristics of a fatigue fracture.

Possible causes for the damage

  • Abnormal combustion, often as a result of knocking combustion.
  • Hydraulic locks.
  • Improper handling of the piston pin during installation.
  • Overloading of piston pin through improved engine performance.
  • Weakening of piston pin through tuning measures (weight reduction).
  • Incorrect piston pin.

Damage assessment

Piston pin fractures are caused by excessive loads. Under overstressing conditions, the deformation of the piston pin into an oval shape in the piston pin bores initially causes a longitudinal crack at the ends of the piston pin. This crack can moriginate both in the exterior surface and on the interior of the bore. The crack then spreads towards the centre of the pin. In the area between the piston pin bore and the connecting rod eye that is subjected to the greatest shear stresses and bending stress, the crack then changes direction and becomes a lateral crack. This then ultimately causes the entire piston pin to break right through.

Fig. 2 shows that an incipient crack may not only be caused by overstressing, but also as a result of improper installation of the piston pin. The end face of the broken piston pin clearly shows that the incipient crack was caused by impact damage (hammer blow). An incipient crack can lead to breakage of the piston pin, even under normal load conditions.

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