For cast iron/steel, the minimum thread engagement is 10mm. (1 diameter). IIRC these studs have about 12mm on the short threaded end.

The Mazda nuts are not copper - they are inconel or high alloy stainless steel. You can't anneal them like copper. I would use a stainless washer.

Dave

 

In that case I would use stainless too.

 

getting back to jimbo, I read through some of his other post if he comes comes back to this post I'd suggest taking dave up on his offer from last month to go over his car.