I own an Atlas/Craftsman 12x36” lathe. I have not done any cylinder or piston work on it, have not cut threads. It is what I would consider a very basic, light weight hobby machine. It does very well with aluminum, brass, bronze, copper and cast iron. It can do steels with a very light touch, good bits and a lot of time. It can produce very good fit and finish.
Cons...
It will flex, easily.
It is not a tight machine compared to others. Cross slide leaves a bit to be desired, works, more a shot gun than a rifle.
Not a fast machine compared to others.
Small pass through on head stock.
Really needs the steady and follow rest, more so than other heavier lathes.
Gears are reputed to fail easily. I have had no issues with mine however have seen a couple atlas craftsman lathes with damaged gears for sale. Check them carefully, especially if thread cutting is important.
Some pros...
3 jaw and 4 jaw chuck are actually very good. Rests work well. Tail stock is basic but functional.
Lots of new and used tooling and parts easily available inexpensively compared to heavier lathes. New import quick change post and 5+ holders are less than $200.00 delivered. Boring bar sets with replaceable carbides less than $100.00. Both are more capable than the machine.
Light enough to be easily moved by hand with two capable people.
Tons of information available.
Not much to these lathes. Easy and inexpensive to maintain, clean, lube.
Personally would not recommend going small than a 12” swing. Cannot think of any common split crankcase cylinders that would not fit and there is enough room to make things easier.