does the 2.5" refer to outside diameter or inside diameter? trying to figure out if a 2.75" will slide over but don't know if I have to factor in wall thickness? 

anyone?

You will find a 2.75 will be way too big to go over a 2.5 without a reducer.

yea but if its 2.5 inside with 1.5mm wall thickeness then its 2.5" + 3mm

But you would still need to find another 3-4mm.

What are you trying to do???

decat 2.75" to 3" cat to 2.5" exhuast

 

or 2.75" to 2.5"cat to 2.5" exhuast

 

essentially I want to be able to remove cat without having to remove the whole downpipe as I've already snapped a stud and rounded a bolt and this will make it easier

 

so 3" downpipe which bolts to a 2.75" pipe which reduces down to 2" but I want to chop it at 2.75" to clamp a cat on or a straight peice so I can interchange easilly

If you are doing this to swap over at MOT time then it would be so much easier to swap the complete downpipe.

If you are swapping them over on a regular basis then fit new studs and bolts and they should be ok for years.  

I was considering doing this when i decatted mine but ended up buying a sports cat instead,

 

The milltek system is 2.5in and designed to fit there front pipe from memory so if you have a standard cat am guessing your already running some sort of expander pipefrom 2.1/4 to 2.5in,

 

A 2.5in bit of pipe will slot straight into the milltek centre section so if you get abit of pipe made up with a flange on it and similar on front pipe then you can get 2 matching ones on a cat and straight pipe as swop as you like,