Chainring. Swap

[Ajax Bay]

have a 68mm/122.5mm Bottom Bracket with 3 gears, I want to change to a single chain ring.

ETA: The new chainring should be in the same place as the middle ring from the old 3 set - this assumes that the current set-up has a reasonable chainline. It does matter (if you don't get the chainring lined up with the centre of the cassette you may have trouble keeping the chain seated), but the rings won't stop turning. The distance between rings (on your current triple) is 5mm. If you're replacing the current triple with 'new' single chainring cranks then the BB should be specified with that (assume square taper).
Have a read: https://www.sheldonbrown.com/chainline.html

 

[ianc6472]

Ajax cool post and link very informative thanks...