Lathe size vs chuck holding size?

[Aaron_W]

Looking at lathes and curious if there is any rule of thumb for lathe specs?

Can a 7x16 lathe turn a chunk of aluminum theoretically 7x16 or slightly under? 8.7x23?

Many times they advertise chuck size in description.
What diameter material can actually be chucked up? Are all jaws reversible so a 4 inch chuck can hold something about 4 inches in diameter?

No, but not in a false advertising sense. A 7x16" lathe can physically mount a piece 7" in diameter, but not one 16" long because the carriage / cross slide will interfere. Similarly it can physically mount a piece 16" long, but at nowhere close to 7" in diameter.

16" is the distance between centers, which specifically refers to holding work between centers. Using a chuck will reduce this by a couple of inches. Using tooling in the tail stock (boring bar, drill bit and chuck etc) may reduce this length by several inches.

The nominal swing specifically refers to holding work with a faceplate and there are severe limits on the work that can be done as the carriage and saddle will get in the way.

As to a realistic work envelope, the between centers distance is fairly accurate for turning, and even facing, but drilling or boring is probably more like 10-12" at most. The swing over the cross slide (a more meaningful measurement) on the 7" lathes is around 2-1/2". Lathe designs do vary so while the 7x16 mini lathes have a practical swing of around 2-1/2" another brand / design of 7" lathe could be a little more or less that that despite having the same swing simply due to the carriage being designed to ride a little higher or lower on the bed.

Also 7x16 (or whatever) is simply a nominal size actual is often a bit larger or smaller.

Same goes for chucks, a 6" chuck is 6" in diameter, some may be able to hold a work piece a maximum of 6" in diameter, some 6-1/4" some only 5-1/2" so when shopping for a chuck it is good to look at the specs, not all 6" chucks are made equal.
Also worth looking at the size of material that can pass through the chuck. Buying a chuck with a 3/4" bore to use on your lathe with a 1" spindle bore means you are limiting your options with the chuck. Rough rule of thumb is a chuck should be 50-60% the swing of the lathe so a 7" lathe will usually be found with a 3 or 4" chuck, a 10" lathe a 5-6" chuck. 4 jaw chucks can often be a little larger than a 3 jaw.
You could put a 6" chuck on a 7" lathe, it will fit and turn, but unless only holding a very tiny part the jaws will hit the ways of the lathe.