I'm messing with several 90's 3X7 mountain bikes. Still original Deore setups. They came with a 122.5 BB. 135mm rear spacing. I think the 122.5 is too long. That's just from eyeballing the chain line angles by eye. I changed one bike to a 118mm BB. I think that centers the chain better on the rear cogs. I'm looking at moving to 3X8. For me a 120mm BB would be what I'd like using with that setup.