Ok... I was kinda judgemental with somebody recently about figuring
out a thread. It bit me in the butt today. I was converting a belt
drive Z axis on a CNC router to a direct drive with a helical
coupler. I needed to replace the pulley on the ballscrew with a
bushing to fit the helical coupler. The pulley was also the nut that
puts pressure on the fixed bearings for the lead screw. Being a
chinese machine I assumed it was metric, but after fighting with it I
don't have a clue what it was.

It definitely was not 8x1.25. I didn't have an 8x1 tap and I couldn't
find my thread gage. My 5/16 x 24 tap threaded easily into the pulley
about 7-8 turns, and then bound up. Got about 6 turns when I double
checked with a 5/16 24 bolt. I figured it was just garbage around the
set screw holes, so I made a set screw bushing to replace the pulley
with a 5/16 24 center taped hole in the middle. It went onto the end
of the ball screw about 4-5 turns and then bound up. I knew there
wasn't a problem with burrs on my bushing as I triple chased each hole
to make sure. The main bore and both set screw holes.

Finally just on a whim I whipped out a 5/16 24 die and it went a 7-8
turns onto the end of the ballscrew and started to jam up. Getting a
little frustrated I went ahead and ran it down. I can just barely see
the tiniest little double ridge under my pocket scope, but its a good
solid looking thread. No wrose than what a rolled thread looks like
under the magnifier.

I haven't a clue what thread it used to be, but its a 5/16 24 now.