MTB crankset change, large ring from 42 to 48 teeth I have changed my front crank set from 42 to a 48 tooth ring. I am having difficulty setting up the front mech. The problem I am having is that the (don't know the name) tail? the box frame bit at the back of the mech is painfully close to the large cog. 
Whilst this is working I have no faith in in it remaining in place, if is does slip the results could be nasty.
So in performing this change is there a specific front mech change that I also need to perform?
with a bigger chainring you may simply need to install the front mech higher up the seat tube.. The outer part of the 'cage', if correctly positioned should be just a couple of millimetres above the biggest chainring when you change up..
Not sure this answers your question but i hope so...!