TDIMeister
Phd of TDIClub Enthusiast, Moderator at Large
Well, first of all, take this for what it is: advice given over the Internet. 
Second of all, engine failures are rarely the result of a single factor in complete isolation; oftentimes it is a combination of things and failure analysis, just like a lawsuit, involves putting together facts to the case that led to the occurrence beyond a reasonable doubt.
It is very interesting to note that the fissure occurred almost parallel to the axis of the wrist pin.
The picture below shows the exaggerated deformation of a piston being subjected to thermal stress. High temperatures cause the top of the piston to thermally expand more than the cooler bottom part of the piston, but the expansion is also constrained by the pin bore and the webs of aluminium that support it. This expansion is therefore not symmetrical around the piston diameter axis but is preferential on the orientation of the piston as shown.
On the other hand, under gas pressure loading, the deformation of the piston crown tends to be 90° from the former case; on the right-hand side of the picture, the stress will want to split the top of the piston along the same axis as the wrist pin.
It would seem like it's absolutely clear that the crack you experienced, based on the above, was caused by gas pressure (excessive PCP), but again -- and this applies especially to pistons -- it cannot be considered in isolation from thermal stress. Aluminium alloys decrease drastically in strength as temperatures increase. Materials developed specifically for pistons (e.g. Mahle 124, 2618, etc.) are designed to better withstand high temperatures, but mechanically-stressed areas should still remain below 250°C, while areas like the bowl lip can see temperatures reaching 350°C.
http://bit.ly/eJodTW
I showed in my paper (page 15, in case the link does not automatically take you there) that piston heat flux scales with RPM and in fact correlates well with engine brake power.
A likely sequence of events is that high temperatures from sustained high-load, high-RPM driving (Autobahn?? ) caused piston temperatures to increase to a very high level, which weakened the material sufficiently that gas pressure took over for final failure as we see:
Second of all, engine failures are rarely the result of a single factor in complete isolation; oftentimes it is a combination of things and failure analysis, just like a lawsuit, involves putting together facts to the case that led to the occurrence beyond a reasonable doubt.
It is very interesting to note that the fissure occurred almost parallel to the axis of the wrist pin.
The picture below shows the exaggerated deformation of a piston being subjected to thermal stress. High temperatures cause the top of the piston to thermally expand more than the cooler bottom part of the piston, but the expansion is also constrained by the pin bore and the webs of aluminium that support it. This expansion is therefore not symmetrical around the piston diameter axis but is preferential on the orientation of the piston as shown.
On the other hand, under gas pressure loading, the deformation of the piston crown tends to be 90° from the former case; on the right-hand side of the picture, the stress will want to split the top of the piston along the same axis as the wrist pin.
It would seem like it's absolutely clear that the crack you experienced, based on the above, was caused by gas pressure (excessive PCP), but again -- and this applies especially to pistons -- it cannot be considered in isolation from thermal stress. Aluminium alloys decrease drastically in strength as temperatures increase. Materials developed specifically for pistons (e.g. Mahle 124, 2618, etc.) are designed to better withstand high temperatures, but mechanically-stressed areas should still remain below 250°C, while areas like the bowl lip can see temperatures reaching 350°C.
http://bit.ly/eJodTW
I showed in my paper (page 15, in case the link does not automatically take you there) that piston heat flux scales with RPM and in fact correlates well with engine brake power.
A likely sequence of events is that high temperatures from sustained high-load, high-RPM driving (Autobahn??
) caused piston temperatures to increase to a very high level, which weakened the material sufficiently that gas pressure took over for final failure as we see:
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