While I was replacing my cylinder studs, I decided to restore the threads by putting some Blue Magic metal polish on one of the old studs and running it in and out of the hole a few times with a cordless impact driver (3/8"). I accidentally backed the stud all the way out so when I went to drive it back in it cross-threaded the hell out of the hole. Others on this forum have said these bolts don't have a standard pitch so I didn't even bother getting a tap and die set. This is what I came up with instead. I may have over-complicated the traditional tap design but it worked like a charm first try. The following was done with a dremel and cutting wheel.
The first thing I did was grind off the first 5 threads up from the bottom of the bolt. I didn't grind them off all the way around, only to the point where threads were not visible on the other side of the bolt (apporx. 90° worth of threads left. The idea here was to be able to slip these threads past the cross threaded part to contact the good threads deeper down.
I then cut off the next three threads all the way around the bolt. The reason for this is so the cross threaded threads won't impede the lower portion of the bolt from mating to the good threads.
With the remaining six threads on the bolt, grind them all around so they're tapered, where the lower threads are almost gone and the upper thread is untouched. Then grind three evenly spaced grooves in this upper thread area.
This tap will allow the small section of threads to bypass the cross threaded section completely. As you start to tighten the bolt down it will center itself and the good threads lower down will be the first to mate together ensuring perfect alignment. My lower threads didn't slip past as I had expected- I had to thread it in but was able to do it by hand so I knew it was ok.
Finally, since the above bolt will only fully repair the very top of the hole, follow it up with a bolt like this. More of a standard tap design with three evenly spaced grooves. Use oil with both bolts.
Now if only I could remove my snapped off cylinder stud
I snapped one of the studs around the #3 cylinder on the intake side. I've heard of other people using that method with mixed results. It seems like there's a little too much of a risk of snapping the bit off in the stud, leaving an even harder metal to drill out. I'm also not confident I can get the bit perfectly centered and straight so this engine is going into the machine shop this week.