Vice screw identify

[mf294-4]

Cleaned out my late fil trucking business. Found a machinist vice. It probably 6-8” and weighs 80 lbs. only problem the threads in the housing is bad. I got some 1” 4 left hand acme nuts from McMaster-Carr and they won’t begin to thread on the screw. What size is it?

 

[mattthemuppet2]

that looks like a square thread profile, not acme. Acme has a 58deg included (both sides) slope to the flanks of the thread

 

[Jim F]

^^^ square thread.
Lots of vises have square threads on the screw,

 

[benmychree]

Besides that, most machinist vises use a right hand thread on the screw and nut.