Bottom bracket question

I’ve slowly been upgrading a trek 420 from the 80s, and I’ve upgraded the wheels and now have an 8 speed cassette, reverse phone lookup and there used to be a 6 speed freewheel.I haven’t upgraded the pcpartpicker older loose bearing style bottom bracket, since I didn’t really have the need to. The question of spindle length has been in my head for the longest time, should I change the spindle length when I do change my bottom bracket?

Assuming everything fits and functions well, why would you want or need to change the spindle length?

What’s your goal in changing the BB? Fit a new crank, or just replacing worn-out parts?

If you are re-using the crankset or replacing with the exact same make/model keep the spindle length the same.

If you are moving to a different crankset get what is appropriate for your chain line.

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