Originally Posted by
Retro Grouch
What have you been tightening it with?
The torque spec for Shimano cartridge bottom brackets is around 30 lb/ft. Imagine a 30 lb weight hanging off of the end of a foot long wrench handle. That's more than I'd probably use if simply left to my own devices. If it was my bike I think that I'd remove the bottom bracket completely, grease the threads and reinstall it tightly.
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+1 proper torque is your friend. To get that 30lb-ft of torque, I'm typically using an automotive type of torque wrench, 1/2" socket, 15" handle length. It's more than most people imagine.