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.

"...should be 20 teeth..."

have you tried using the standard 20t tool yet?
and i've not been impressed with the longevity of "sealed" BB's so far... most last about 1/4 the time of a good quality "loose ball" BB... my shimano 600 BB had about 20K miles on it before i needed to replace the balls and spindle... mostly because i didn't change grease for the last 5K or so... it has SEALS in the cups... the tiny bearings in those internal, more heavily loaded, bearings are the wrong type of "seals", and the nature of that design places higher loads on the bearings due to their farther in location on the spindle....... they're called "Dust Covers" in similar "sealed" bearings, btw... and most "sealed" bearings are sold with little to no grease in them.... i always popped the seals out and repacked them before install in various ag. equipment applications.... not an option with the internal BB's i've seen...

Thanks for the input. Now I just have to figure out where I get one locally around here.

Any bike shop will have BB tools. Make sure you have some washers to keep it in place, sometimes brute force is required.

I just tap on the wrench with a 3 lb hammer until it gives, some folks like the persuader method but I am patient.

If you know this this already I apologize, I can't tell if you are a novice or experienced guy.

https://www.bing.com/search?q=Los+Al...s=ds&form=QBRE

the internet is your friend.... look for a community bicycle Co-Op in your area...

The local Performance store has one. I"m going to hit them up in the morning when they open.

I'll tell you one thing, I was really surprised how bad the BB felt once I removed the crank arms. The bike has always been taken well care of. I re-grease the hubs and headset about once a year but never bothered checking the BB. With the crank-arms on, it felt smooth. The non-drive-side crank arm came off really smooth and as expected. The drive-side required a little "percussive therapy" with a hammer on the wrench handle.

Thanks again for everyone's suggestions and help.