Ignoramus18195 wrote:
For example, can I swap a step pulleyed head, for a variable speed
head? Are the mounting arrangements the same?

1J (step pulley) and 2J (varispeed) are interchangeable, in fact the
main casting is identical. The ancient M head is NOT the same, however,
a much smaller bolt circle. As long as it is a V-ram or rigid ram
machine, it would be unlikely to have an M head. The 1J has 4 belt
speeds and a geared low range, with a toothed reduction belt below the
vee belt. It also has power down-feed with a 3-speed selector. The M
head has 6 speeds, no back gear and no power quill feed.

One other thing, various J heads have different spindle tapers, mostly
your choice of R-8, NMTB 30 or a couple quick-switch systems.

There are the 3J and 4J heads on Series-II machines, definitely NOT
interchangeable with 1- and 2J. (Note, Series-II tracers and
multi-spindle machines often had several 2J heads on them.)

There are also BOSS heads which have a coaxial ballscrew machined into
the quill. These are CNC-only, no provision for normal manual quill feed.

Jon