1/2-13 on the logan looks drunk

Trying to cut 1/2-13 for a drawbar nut. The thread gage rocks. It starts ok and then the pitch looks stretched by the time I'm at the end of an inch. Gearbox is set where the plate says 13. I did not disengage the half nuts. I'm going to bed angry unless somebody tells me the plate is a liar.

If you never opened the half nuts, the gearbox didn't 'drift.' Either the lever isn't seated in 13, or the tool is springing and the crest looks drunk while the pitch is fine. Check the thread against a known nut, not only the leaf gage. A leaf will rock on a rough crest and lie to you.

Try the nut. The gage is for people who already cut a decent thread. Also your tool is probably sticking out too far if the crest is getting worse as the depth increases. That's spring, not pitch.

The nut goes on. The gage was rocking on a torn crest. I shortened the stickout and took a lighter pass and it looks like a thread a person made. The plate was not the liar. I was. Leaving this up because I searched 'drunk thread' and got nothing, and that's what I typed.