Your cranks and chain didn't break your BB. If it's only a month and a half old, the BB was either defective or improperly installed (assuming this was a new bike at the time), and the shop where you purchased the bike should fix it. Can the guy at the bike shop explain what's so crappy about your chain or cranks and what's so wonderful about 1/8" for your bike and the way you're riding it? Sounds like he's feeding you a load of BS so you'll waste cash on unnecessary upgrades rather than just fixing the $20 part that's shot.