I didn't snap a bolt but I had the same experience. Those long bolts fit fairly snug in the thru-hole in the alternator, and in my case the bolts had a buildup of hard corrosion inside the hole, preventing them from coming out. I finally had to remove my radiator and A/C condenser to be able to whack the long bolt out from the front, using a big hammer and socket extension.

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'81 300CD - 180K, cannot be killed by any conventional means
'99 Ford Escort - good MPG