I was taking my Trek 7.4 FX apart for a thorough maintenance and cleaning today and once I got the Square Taper crank arms off (first time they've been off on this bike) I could feel and hear that the sealed cartridge BB was toast. This bike couldn't have 2500 miles on it and the BB is shot.

Ok, now for my question... the teeth on the BB shell are in groups of 4. In between each group of 4 there is a space where you would expect to see another tooth. I've never seen a configuration like this. It appears there should be 20 teeth but there are only 16. Which tool too I need? I'd like to get it pulled so that I can confirm the size of the replacement BB I need to buy.