Crankshaft Replacement So I have an old Schwinn Crossfit MTB that Ive loved and had for a long time and recently wanted to fix and upgrade it a bit. I wanted to know about the crankshaft compatibility for it, as ill im able to figure out is that its a 28/38/48 Shimano Tourney, and I was curious if it was compatible with the newer ones of the same brand and name. Yes this is not the best bike, and its also not my only bike, but its more a sentimental thing and a good opportunity to learn more about replacing and figuring out crankshafts for more experience and future work. Thanks so much for your feedback!
'Crankshaft' if not the name of a bike component and is somewhat ambiguous (some people don't know the proper component names - no offense meant by pointing this out). You could be talking about the bottom bracket, which is the spindle connecting the cranks and its bearings; or you might mean the cranks themselves.

It's probably not worth upgrading components on an older inexpensive bike unless they are worn out or broken. Such a bike can benefit from a good servicing though.

On an older Tourney equipped bike you most likely have a bottom bracket with a square taper spindle that the cranks attach to. The bottom bracket will be an adjustable or cassette type.

On an older bike the bottom bracket may well be worn. If it does not spin freely or there is any play in the bearings it's worn (or needs adjusting). There are some guides available for servicing or replacing adjustable and cassette bottom brackets. Be aware you'll need special tools for removing the cranks and adjusting or removing the bracket. If you need to replace a cassette type (or replace an adjustable with a cassette) you need to know the bottom bracket shell with (68 or 73mm) and the spindle length. The shell width and threads are standardized.

If you are thinking about replacing the whole crank, consider replacing just the chainrings as they are the part that wears. If you really want to replace the crank you need one that is for the square taper bottom bracket. Any triple crank for square taper should be compatible.