Measure your chainring position relative to the seat tube to confirm you're currently at 50mm CL.
What size bottom bracket do I need?
For a standard MTB setup a 113mm axle is needed for ISIS or square taper.
Middleburn cranks run a 50mm chain line for cross country and downhill.
Measure your chainring position relative to the seat tube to confirm you're currently at 50mm CL.