No experience with E90s but lots of experience with E46s (x3). I have never had headbolt issues but have heard discussions of them - seems to be related with over heating the engine and warpping the heads. I just drive around with a code reader and pack one for the track and longer trips. The cooling systems can be an issue if the system is not replaced at approximately 100k. Replace the whole system at once as the cost is 500$ for the complete kit and your good for 100k. Do not replace individual parts or your life will not be worth living. One of my e46s has 190k and 50 track days and the compression is still 185 to 195 psi. These motors are tough but do not over heat them.
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