In a message dated 11/28/04 2:42:19 AM, jeff@VANAGONPARTS.COM writes:

<< The simple fix would be to drill out the hole, retap to 10mm and use a

step-down stud, 10mm to 8mm.

A better fix would be to use an insert and keep the 8mm stud size.

 >>

A stepped stud is a decent fix with one caveat: in deference to any future
mechanic, clearance the hole in the header flange so that 10 MM will pass
through it easily.  If the nut becomes stuck to the stepped stud, not necessarily
unlikely for an exhaust nut, it is possible to strip the now enlarged tapping by
trying to back out the stud through a flange hole which is too small.
Certainly a rarity, but I've seen it happen.

An insert is the better repair and I have come to favor the keyed variety.

In a pinch, there may be sufficient length in the deeper, partially
tapped/untapped portion of the original tapping to complete the tapping with a plug tap
and thereby provide enough threads for a bolt up.  The rule of thumb is that
a tapped thread length of one and one half times the fastener diameter will
provide sufficient pull out resistance to the maximum torque rating of that
fastener.

George