Even though you can often simply adjust a BB unit to fix problems, any sensible mechanic should really be opening the BB unit (if possible) to inspect for hidden damage. This will more often than not give a better idea of what is required.
WORKSHOP TIP: If the bearings or bearing surfaces are damaged or worn, or indeed if there is simply too little grease or too much dirt in the unit, no amount of adjustment will make the unit work. In which case, a refurbishment or replacement will be necessary. Also, it may be the case that unevenly worn parts will cause increased wear to any new part, not really fixing the problem or making the repair worthwhile and making total replacement of the BB unit the only feasible option.
Many open/loose bearing(adjustable) BBs can be refurbished by replacing worn bearings. Replacing the balls is easy and cheap, and can give a previously well-adjusted part a new lease of life.
You will always need to replace sealed units if they are worn. (Apart from with some expensive cartridge BBs where it is possible to drift/replace bearings.)
On outboard bearing units however, you will only need to replace the bearing cups.
You replace the bearings on these units by drifting the sealed bearings out of the BB shell and pressing the new bearing unit back into the frame.
At some point it is worth asking whether the BB is also worth refurbishing. It is important to note that BBs are sometimes very difficult to remove. In a commercial environment, the cost of the repair should be anticipated and the customer informed. It may be that working on BBs of very cheap modern bikes is uneconomical.It
WORKSHOP TIP: Working on an old bike is not necessarily going to be as difficult as you might assume. A quality old bike which has been well maintained (most importantly greased regularly) can yield its BB more easily than a cheap new bike. (This is due to the fact that cheap BBs are factory installed often with too little or with cheap grease).
Spindles
There is no point replacing the parts for an open system in a bike shop as the labour charge involved would be more than the total labour and parts costs for replacing a whole new BB. The only time you would choose to replace a spindle to repair a bottom bracket unit would be if you were restoring a period bike.
Threads
If you remove the BB and find that the threads in the shell are damaged, the only option you have is to return the frame to a frame builder for repair. If the spindle threads are damaged, you can replace the spindle in order to repair the bottom bracket unit, but this is rare. Much more economical and easier is to simply replace the BB unit.
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