If the threads won't clean up with a tap use a heli-coil.The finished job will actually be stronger than the original thread! We used to install them regularly on aluminum race heads to keep rocker studs from pulling out of the aluminum bosses.

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82 300SD 202K
82 300SD 233K
83 300TD 340K
82 300TD 98k euro (parts on the hoof)
85 300TD 282K
83 300TD 197K