XLR8R

This stuff isn't black magic - what do you think the Top Fuel & Top Alchohol crewchiefs are having the H11 studs in their mills torqued to? 625s are just an expensive stainless version of tool steel.

More than likely, thread form integrity was compromised in the offending hole - it can happen easily, either during the bottom tapping or installation. This is the procedure I use:

1. Blow holes out clean with compressed air.

2. Run a plug chamfer tap to the "bottom" by hand with tapping fluid.

3. Repeat #1

4. Repeat #2 to the actual bottom of the hole.

5. Repeat #1

6. Run the bottom chamfer tap to the bottom by hand with tapping fluid.

7. Repeat #1

8. Thread dry stud down to bottom of hole - check depth.

9. Repeat #1

Some may think this is **** retentive & a waste of time... but I've been winning the war against Murphy's Law for a long time now!

I prefer to tap with the head off, since QC is so much easier.
A good cutting fluid for cast iron is important.

I like to use the tightest class of fit for the thread form available in a given tap size, since it avoids excess cutting action/wear on the existing threads. For example, an H1 or H2 will leave more material in the block than an H3 or H4 class fit tap.
The additional friction presented by the tighter threads at the bottom of the hole helps to keep the stud from augering in while torquing the nuts.

Also, proper assembly lube and the application thereof is really important, since only about 10 or 15% of the torque is used to generate a clamp load - the rest just turns the fastener against prevailing torque... that's why it's also a good idea to gently pull those torque settings. Once the stiction threshold is reached, it's much easier to bypass the desired torque value with a fast swing.