I have found the female threads in the engine relatively easy to strip if you are not careful to stop tightening when they are snug. I have always though the bolts are plenty long enough to reach the threads, so it may be possible that a previous owner or mechanic may have stripped them.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record