I haven't read more than the first page but can make a few comments. I have a Grizzly G-1550, a Taiwan version. The current Grizzly is a G-4000, made in mainland China. The G-4000 is simply a clone of many 9X19s, they all being essentially the same. A Jet is a cleaner(?) Taiwan version, like my G-1550. Having purchased a HF version when times were flush, I acquired it for extra parts for my contraptions. The cost of the HF as a unit being, at the time, less than all the parts I wanted from Grizzly. Overall, the only difference I ran into was the spindle thread. The Grizzly G-1550 (and JET) are 1-1/2X8 where the Grizzly G-4000 and all HF machines are M39X4.
There may have been a few fasteners that were different, but I never ran into any. The change gears were the same across the board. I did have to "tweak" the inside of some of the HF gears. But that to get them to fit their own banjo, not for interchange. I gave away the HF machine (less a few parts) some 20 odd years back. The biggest reason was the spindle threads, M39X4 versus 1-1/2X8. The guy that recieved it was happy to get something, anything, he could learn on. He may have replaced the parts I kept, he may have dropped the whole idea. I haven't talked to him since.
All of these machines that I have had experience with cut threads, both imperial and metric. There is a 127/120 pair of gears permitting metric. The gears are modulus 1, the 127 tooth being about 4 inches diameter. Any gears missing in a "used" purchase can be acquired from Jet, or my preference, Grizzly. Grizzly has dropped the 1550, way back in the mid '90s. They no longer have the 1-1/2X8 threaded devices. But such can still be found. The HF machine is M39X4 anyway, so parts can easily be found at Grizzly.
The lead screw is a 16 pitch (almost but not quite) ACME. I think, but not sure, it is a trapezoidal form, common on Asian machines. It can be thought of as 1.6mm(?) pitch as well, it is VERY close. There is a Half Norton change gear box, ranging is done by changing some loose gears on the headstock. This is also where the metric conversion gears fit. All in all, after fine tuning, a quite usable, if limited, machine.
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