Bike Mechanics - How can you tell if a sealed BB is shot?

How many miles on it?  After about 2,500 - 3,000 (as a very rough number) I would say it is in your best interest to replace standard Shimano ones.

The creak just means you need to pull it, lube the threads and put it back in snug.  The "thunk"??  Not sure about that.  Perhaps the bearings are shot.

Just hold onto the new one and you'll have your back-up when needed.

Does it spin smooth without changes in resistance when you have it out of the bike?

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It all depends on the conditions you are riding through.  If it is crappy out and you ride a lot, be prepared to replace them often (yes, even cartridge sealed ones).

It is very easy once you've done it once or twice.  Or you can go back to my Crank Install thread and get step by step.

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