I need to replace the bottom bracket shaft and bearings.when they sell the shafts they are in mm lengths.is that the lenght of the entire shaft? Or from the end of the threaded end to the other side sort of the middle of the shaft minus the threds thanks for any input

On the basis you have a square taper, you will need 2 measurement, the width of the shell, and the whole length of the spindle (shaft), examples of this would be 68mm or 73mm for the shell, 68mm is standard for road bikes, 73mm is common for MTBs, and the spindle length will be say 113mm.

Threading will probably be english, as this is the most common, but there are various others, check the Park Tool link below for more info

If you can remove the Bottom Bracket, the info you need may be printed on it (Shimano have the spec info on a foil sticker)

For info about other types, see here https://www.parktool.com/blog/repair-...ottom-brackets

If you are servicing a traditional cup-and-cone bottom bracket and you need to replace the spindle, there will often be a code stamped into the spindle, something like "3K". This page has a database of Japanese spindles as well as information on other types of BBs: https://sheldonbrown.com/bbsize.html

If the spindle (aka the shaft) has threaded stubs that take nuts at each end, you measure the overall length between the square bases of the threaded stubs, i.e. you don't include the length of the stubs in your measurement.

Better quality spindles do not have these stubs and the ends of the square tapers have threaded holes that take bolts. These you measure end-to-end.

End to end, minus the threads.

thanks guys its just a junker my son can take to college.this way when they rip it off i'm not out

Also, make sure you don't have an asymmetrical BB shaft. Some bikes had shafts where the distance between the cones and ends of the shaft were different on the two sides. If you have that type and just measure overall length, you will mess up your chain line.