Also need to know the age of the Stainz, Dan - the early ones (clamshell gearbox, metal motion) have a long threaded shaft on the stack, the later models (sandwich gearbox, plastic motion) a shorter one.....
These are the newer Stainz locos with the plastic motion. It does not need to be a smoker. I've given up on smoking with the exception of one 2017 type loco that I use for when guests want to smoke. Smokes got too expensive to replace too often.
Hmmm, you may be right there, Phil - it's been a while since I've had a newer-type Stainz apart..... Thinking about it further, the newer ones have a square-section plastic section at the base of the stack itself, as I recall, which the older ones don't. Maybe best to stick with the "correct" stacks for the period of model - but you should still be able to get straight versions, I think?
Another thought, some folks make straight stacks by simply cutting the large spark arrester away, leaving just the inner pipe - though this does look (to my eyes at least) a little odd, since the stack then tapers gently from the smokebox end but then abruptly goes vertical, because that's the shape of the inner pipe "core" inside the large spark arrester. I have a couple of Stainz conversions that have had their stacks done this way, and I'm not particularly happy with them, though their builders obviously thought it looked OK!
I wanted a straight stack for a newish Stainz too. I wasn't bothered about it smoking so I picked up one of the old non smoking straight types as fitted to the older Elias steam trams (And probably others too.) They are an M6 thread and a shank that's shorter than a smoking version. (Both old and modern.) The stainz I fitted it to is a 20212 (From a starter set.) which had the ballon type funnel with the square section mount. In order to fit it I did the following; Sourced a front lamp with the round hole in the bracket from an earlier stainz. Used a Flat headed M6 cabinet joiner shanked nut to give me the correct length. Bob's yer uncle. Looks Ok and nice and secure. See photos. (Both of these are 20212's.) Green original funnel, black modified. NB; Any suggestions of a more suitable colour scheme for the plough?