I've owned a 9x19 Grizzly lathe (mod 4000) for several years and have never had an occasion to cut a thread on it until yesterday. I got the Grizzly book out and finally deciphered the 7 words in it that deal with cutting threads. I changed the "A&B" gears to 30 teeth each, set the speed to 150 rpm, chucked up a 1/4" rod, set the 60 degree tool to cut .005", set the lever to gear 4, engaged the 1/2 nut and turned the thing on and made several passes, Well, I was really impressed with myself, having made a thread that looked like it should accept a 1/4" nut. But it didn't work out just that way. When I checked the new threads against the threads on a commercially made 1/4x20 bolt I found my threads to be 21+ threads per inch and no way a 1/4" nut would go on. Is it me or the machine? Any help will be appreciated.  Also, my book shows a picture of a "center gauge" with a bunch of fractional numbers stamped in it. What do the numbers represent? Being old and stupid has several disadvantages. These are just a few.  (Don Greife)

Sounds like you paid proper attention to the change gears OK.  But take a look at the 120 and 127 tooth gears.  One is for cutting metric threads and the other is for S.A.E. threads.  What you may have done is cut a metric thread.  Only a suggestion.  (Jerry Young)