I'm replacing my rear cassette from a 13-25 to a 12-27. Replacing the chain and large chainring at the same time too. Due to the change in the new cassette range, do i need to change my chain length from it's original no of links or do I keep it the same?

Cheers

You will need a slightly longer chain. Use one of the sizing methods on the Park Tool or Sheldon Brown's website. The easiest thing to do is to wrap the chain around the largest cog and the largest chainring, and add one full link to that length.

Slap it on and go. Couple teeth are no big deal. I routinely swap cassettes from 12-25 (training) to 11-21 (racing) with no ill effect. Unless your chain was on the short end originally for the 13-25, the 12-27 will be fine. You shouldn't be crosschaining in the 53-27 anyway.

I'd check it anyways if I were you. There's no guarantee your current chain was correct length to begin with.

Absolutely. Cross chaining, particularly big-big, is not recommended but the chain must be long enough to allow it. An inadvertant choice of that gear can do a lot of damage if the chain is too short.

So what's the big deal? With a new cassette and new chainring I assume you are getting a new chain anyway because it would be really dumb not to.

Wrap it around the big/big combination bypassing the rear derailleur. Add 1 inch of chain. Done.

+1

IF the old chain was the correct length there should be no reason to make the new chain any different. The difference in a 13-25 and 12-27 should not require a chain length change. The two tooth difference between a 25 and a 27 is very small, but one length of chain is an inch, that's a lot.

Al