How to check bottom bracket size

I am looking at investing in some Profile cranks, but I need to know what size bottom bracket I currently have.

Also, if anybody could tell me the bottom bracket size of the GT bump 06 then that would be more useful.

One more thing - taking the bike to a shop is not an option as the nearest shop is several miles away and like to rip me off. Like, a lot.

http://www.bikepedia.com/QuickBike/BikeS...

Unfortunately this bike had 2 different sizes- American is the large diameter that looks like it could fit a one piece crank (and it would). I don't know if this same BB was used in UK models or not.

I would email danscomp.com and ask them what they suggest.

Um, both Profile and GT used Lasco to make their cranks so unless there is a distinct upgrade you probably already have "Profile" on there.

It looks like a loose ball type. Can't tell you the size but it is the 8 spine type that should be standard. You will have to remove the crank arm to be sure. See the link below you should not need the crank puller.

there's only two size the american used on almost all bikes and the italian which is used on the top end italian bike if your bike was built by a non italian company chances are it's an american bb