Italian or English BB

Most European frames have an Italian threaded BB shell, 36 mm X 24 tpi (threads per inch). Most new frames in production have the British or English thread, 1.370" X 24 tpi.

So if you have an English BB, but an Italian threaded shell, the BB is not going to fit because its smaller than the shell. Its simple. Its going to fall right through. FSA, for their crankset range, sell special cups that you can fit to the BB to go from English to Italian.

However, the only downside to frames with Italian thread is that since both sides are Right Hand threaded (or RH), so the left side will unscrew itself over time and needs the additional retorquing. This is what I was told by someone through personal experience.

So never mix and match as Sheldon Brown says. Buy a frame before you buy the BB, or chances are you'll be frustrated with your purchase and end up selling, or you will destroy the frame :)