I'm new to the forum but have read the faq and spent many happy hours reading posts but can't quite find the answer I'm looking for.

I've got a 2006 Sirrus hybrid and have done around 3000 miles on it. Over the last couple of weeks I've noticed a nasty clunk when pushing hard on the left peddle. Further investigation shows that there is a large amount of lateral movement pushing the crank arms towards the seat post. The two arms are solidly locked together and move as a unit - it isn't that one arm moves and the other doesn't. My wife has an identical bike and hers has no lateral movement at all.

Does this mean the BB has worn? I can't hearing grinding noises that I've read are a sign of a worn BB but maybe I am not listening carefully enough.

Assuming I need a new BB, what do I buy? The spec for my bike is as follows

CRANKSET
Sugino RD 48x38x28T
CHAINRINGS
48Sx38Sx28S, Super Shifter III, with chainguard
BOTTOM BRACKET
TH Sport, square taper, 68mm shell, 113mm spindle, sealed cartridge

I've got no idea what "TH Sport" is - I can't find them as a manufacturer. However, I can for example get a Shimano UN54 68-113mm easily enough. Will that be OK or is there more to buying a BB than meets the eye?

Thanks
Darren

Depending on the type of bottom bracket it's either worn, or simply loosened. If you're lucky it's the latter, but in any case a trip to the LBS is warranted, and sooner rather than later so as not to cause collateral damage.

BTW welcome to the forum.

Assuming the specs listed are correct, it is a sealed cartridge so will need to be replaced. The one you found would work fine. If you plan to replace it yourself, you will need a crank puller (to remove the crank arms) and a cartridge BB tool (to remove the BB). Not a difficult job unless you try to do it without the proper tools.

+1 Toss the BB and replace. The UN 54 is a nice one.

My first thought would be to tighten the BB. That is the most likely cause. If that does not help then the UN-54 is a good bet.

TH (Tien Hsin) is a generic taiwanese manufacturer of a great number of crappy OEM parts (bearings, bb's, headsets etc). Unless the BB is loose (this will be evident upon a simple inspection), then it's very likely it's worn out. Like post #4 said. Replace with a shimano Un-5x unit. MSRP is about $25 CAD on that.

thanks very much for the welcome and all the replies. I'm pretty certain my specs are correct - they came directly from the Specialized site. So I'll go ahead and get the Shimano replacement, I also have the required tools and been looking forward to using them all.

I'm not too surprised by that the bike has a couple of cheap TH parts - it is the absolute base model. So far though, the bike has been great and this is the first out of the ordinary issue.