new thread on bolt

Hi there
Apologize in advance for the non technical language. I have got an M12x65 socket caps that is partially threaded. I use a M12 die with the same pitch to fully thread it. The problem is that the nut is way too tight on the new threads. I checked the bolt diameter of the existing threads and the diameter for the new thread that I did. The diameter of the new thread from some reason slightly wider.
My question is what is going on? Why that happens and is it because of that the nut is too tight? Lastly how do i sort it out?
Thanks in advance
Ronen

Sounds like the die you used is low quality or worn out.

Many threading dies have a gap that allows the die to be closed a bit by the set screws that hold it into the die holder. Does your die holder allow this adjustment?

I think too that bolt and nuts are usually thread cut before case hardening so when cutting more threads tends to less accurate.

You may need to die or tap the threads several times.

most threads are actually rolled. The threaded section was probably about pitch diameter when the part was headed and then the threads rolled on that section. You are right about most being threaded before hardening or carburizing.

As I suggested in the other forum in which you posted, buy a fully threaded bolt. They're available lots of places (McMaster. Fastenal) and are not expensive. No need to change anything, and its the very same thread form from one end to the other.