I recently had to replace the crankset (essentially arms and chainrings) on my mountain bike & I find that the inner chainring is now too close to the frame to allow the front derailleur to adequately line up with it (regardless of how its adjusted). My micrometer tells me that the new chainring in 5mm further in toward the frame than the one that it replaced. My assumption is that I'm going to need a new bottom bracket with a spindle that is 10mm longer than the current one (assuming that the additional 10mm of length is spread out evenly over both sides, that is). Does the notation on that old BB that reads BC 1.37x24T refer to the threads & is that "standard"?

Thank... Brian

1.37 x 24 only defines the diameter and threading of the bottom bracket and has little to do with chainline. Another parameter is spindle width which partially defines how far in or out your crank will mount. Assuming that your crank uses a square taper spindle, the taper may be ISO or JIS (or something else) and this may affect the chain line. Best advice is to use the crankset manufacturer's specification for spindle width and taper. If your current BB does not meet these specs, you may be able to correct with spacers (but 5 mm is a lot) or replace it.

When we replace the crankset of a repair we explain that as much as we try to anticipate the results the bike world in not a perfect place of info and meeting specs. We say that only with the actual fitting of the crankset will we know if the OE BB will be a suitable dimension. Of course we know how to estimate the OE BB length with the OE cranks on the bike, we know that the BB condition might motivate us to suggest a replacement (and we try to get a feel of this at assessment) and we know how to read Shimano, QBP and other published specs for crankset/BB claims. Still there are a number of the dozens of service jobs we do that don't work out like we might wish. Having that worst case discussion with the customer up front is always the best approach.

The OP give just one aspect that can be overlooked or not judged well up front. The ability for the frt der to readjust it's movement range to play nice with the new crankset's actual final position. Andy.