Bicycle Mechanics - Shimano 6 Speed

Older Specialized hybred with six speed drive train. Sram shifter, u/k shimano 6 speed freewheel, shimano tourney(?) RD.

I believe I can run almost any 6 speed freewheel and have things work. Sachs, Sun Tour, or shimano.

Is it this simple or do I need to consider other variables?

Thanks, dgsca

If you go with a freewheel with a large cog that has many more teeth than your current one you may have some issues with the RD. Only other thing you might want to consider is the type of removal tool it uses. There are a few different ones and if you already have one it may make sense to get a freewheel that uses the one you've got for future removal.

I'm going from a wide range freewheel to an almost straight block. 13-21

Then you're down to the freewheel removal tool. Make sure your new freewheel is standard english threading (which is the default)

The Tourney der is typically used with the lower end bikes with the 34T large cog, so I wouldn't expect any problem with large cogs.
That said, since the Tourney tends to transcend a cog range of 20 (or more) teeth, it's "B" setting may not be optimal for a corn cob type cog set. I don't think it even has a "B" adjustment screw????
It should still work OK IMO.

Shimano 6-speed freewheels were built to match early SIS indexing and were evenly spaced. Sun Tour 6-speed freewheels were either intended for friction or Sun Tour's Accushift indexing and were not spaced the same as Shimano.

Any Shimano 6-speed freewheel will work with your SRAM (indexing, I presume) shifters. The other brands probably won't.