they thread into the block a single turn or maybe 1.5 turns, and then they bind, turning them further would start tapping the existing threads and enlarge them, and they are already quite loose for the studs.
My assumption was that when deepening the threads to reach the bottom, with thoroughly cleaned original thread, a tap would more-or-less freely thread into the existing thread, go down an inch or so i.e. to the depth to which the threads have been originally fully cut by the factory, and then start engaging as it begins to cut the new thread on the previously unthreaded bottom part of the hole.

My feeling is that if I just tap them like that I will enlarge the existing threads and the overall strength will end up being significantly worse.
Am I doing something wrong here? What is the correct tap thread limit to use here? Or maybe there is another detail about the tap I'm missing?

I would really appreciate your feedback!

Thanks
Emil

Hey David,

Sorry to bother you with direct messages but I figured that if I start a new thread I'll get 10 different opinions. I'd rather ask a guy who I have been listening to for weeks even though he doesn't know it (i'm reviving an old D250, and turns out you had tutorials here for most of my problems, most recently - a head gasket replacement).
So anyways, I'm in the process of replacing my cylinder head, I'm trying to be very precise and not cut corners so I got head studs, all genuine cummins gaskets, machine shop checked off on the head surface valves etc.
Now it's time to bottom tap the block so that the head studs can sit there properly, and since it's my first time doing this I don't really know what to expect (and its really easy to ruin the block so I'm overly cautious).
I got two high quality USA made M12x1.75 bottoming taps, one a "D3" and one "D6" (couldn't find a lower "D" than 3). Both of the taps are measurably larger than the bolts, and large enough so that

https://www.dieseltruckresource.com/...s-tape-328825/

10 years late- but noticed you asked. THey also make a high heat version.

My truck will never be as nice as yours- but I will say THANKS!! I look at good threads to help me- and it's usually written by you.

M