I have seen aftermarket plastic fans explode, but not the OE fans unless they had previous damage. This of course doesn't include foreign objects being in the mix. The fans draw air through the alt from the rear to the front, so drawing something through the alt would be a rare occurrence. The plastic fans do cool better. The extra cooling provided by the plastic fans on the 12SI units was needed because of the extra heat generated by the higher output of the 12SI alternators. The diode assembly design was also changed for better cooling. The front housing of the 12SI is also different. It has a lip around the top to work with the plastic fan. If you put a metal fan on it will need to be spaced away from the lip which would make it even less efficient than a 10SI cooling capability. You can change the front of the alt to a 10SI front to use a metal fan and have the correct spacing and pulley alignment but it won't cool as well as the plastic fan and 12SI front housing. The drawback being the alt won't last as long. It will cause the alt to overheat and burn up the stator and or diodes prematurley.
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