bottom bracket upgrade
 

Hey all. I'm fixing up an '81ish Miyata and found (not surprisingly) that the original bottom bracket is toast. I'd love to upgrade to a cartridge type sealed bb, but am having a hard time figuring out the correct length for the new one.

Here are the dimensions of the old spindle. It is asymmetrical.
Overall length: 116mm
Drive side (shoulder to tip): 33mm
Middle (shoulder to shoulder): 56mm
Non-drive side (shoulder to tip): 27mm

Are new sealed bb's symmetrical? If so, it seems like the length that I would need is ~122mm (33+56+33). But that seems really long considering it's just a double crank. Anyone have insight?

I upgraded my 1986 Miyata 1000 with a Shimano un-52 122 mm and it works fine.
Gordon P