BB Shell Interface

There are two basic types of bottom bracket shell interfaces; threaded and pressfit. Within each of these groups there are many different versions.

For example: there are many different thread pitches and diameters so just because a bottom bracket shell is threaded doesn't mean any threaded bottom bracket will fit.

Almost all bottom bracket shells were threaded for the longest time because they were reliable and easy to install. In an attempt for manufacturers to cut manufacturing costs pressfit interfaces were introduced because tolerances could be looser which would save money during manufacturing. Unfortunately each time a bottom bracket is pressed into a pressfit bottom bracket shell the tolerance of the bore is reduced which results in loose bottom brackets that make noise. 

For example: both the bottom bracket and bottom bracket shell have to have a perfect tolerance in order to avoid damage. If a pressfit bottom bracket shell  measured exactly 41.00mm in diameter from the factory and a bottom bracket measuring 41.05mm is installed then the bottom bracket shell of the frame has now been enlarged by .05mm. Now, in the future when the bottom bracket wears out and needs to be replaced if it measures any less than 41.05mm it will not sit in the frame securely resulting in creaky noises.