Which Rim Size will Fit My MTB? I just got an old MTB, I need to replace all the parts except the frame since the parts are in broken condition, except the sprocket and cassette
I'm a total newbie and LBS here cost crazily high, so I'm trying to replace all the parts by myself
The o.g parts are :

Shimano tourney SIS 3x6 - rusty and broken
Tire size is 26x2.125&quot;
Crankset is rusty and broken too
Brake is also rusty and broken

So the question is

If I replace the group set to - let's say - Micro shift 3 x 9, will the group set work well? Do I need to change the cassette or I can stick with the original one which only have 6 speed and I can't/mustn't shift to 7th gear?



Which rim size will suit the tire? Is it also the 26 inch rim?
If I want to use narrower tire, 26 x 1.5&quot;, do I need to find a special rim size which will accommodate the tire width or any 26 inch rim will just do fine?

Thanks
Just replacing the groupset is not enough. 9 speed cassette has different spacing from 6 speed, so the derailleur steps won't match the cogs. For 9 speed cassette, you'll need a new hub, and the new hub is wider than 6 speed so you need to spread the frame too.

There are many different 26&quot; sizes, none of which actually measures 26&quot;. For 26 x decimal inches, the correct one is 559mm

See above and What is the maximum or minimum tire width I can fit on my bicycle