Riding with a loose bottom bracket can cause damage. One way of noticing a loose bottom bracket (BB) is a clicking/clanking noise, another is slight play when you move the crank arms in and out.

[edit] Fixing it

[edit] Tools needed

To tighten a loose bottom bracket you will need:

• Bottom bracket tool (examples here and here - the one you need will depend on your bottom bracket; look at the teeth to see if a tool will fit)
• Large spanner - generally requires 32mm (adjustable will do)
• Crank extractor

[edit] Procedure

First, the left (non-drive side) crank must be removed. See main article here: Removing cranks.

This assumes you are using a 'standard' square taper Shimano bottom bracket, or ISIS. Other brands may require slightly different tool - For example the Monty bottom bracket uses a different tool. So, fit the bottom bracket tool into the bottom bracket. Attach the spanner to the BB tool, and turn it to tighten. Beware - the non drive side cup (left hand side if you're riding the bike) turns clockwise to tighten - as normal - but the drive side (right hand side) is reverse threaded, meaning you need to turn it ANTIclockwise to tighten. Make it hand-tight, do not use excessive force as this may strip the threads.

Tip: If you are having trouble, you can secure the BB tool onto the bottom bracket by using either the crank bolts (if they are big enough) or a quick release threaded through the BB axle.

Finally, replace the cranks. See main article here: Tightening cranks