2½" -8" have the same threads. 8TPI, tapered at 3/4"/ft (or 1/16"/inch)
Those threaders use one size of dies and cut 2½", 3", 3½", and 4" pipe threads.
We still run into 3½" pipe.
We would use them on the pipe clamped in a pipe vise. The power was supplied through a bar about 6' long with 2 u-joints, with the other end in a 300 machine.
Old fitters would talk about being careful because if the bar with the u-joints bound up it would flip the power machine, the vise with the pipe or both. The mayhem would stop when the set-up had flipped far enough away to pull the cord.
Now all the power machines have foot pedal switches.
Chilly