Just put a new chain and front chainring on my road bike, and after a test ride it’s slipping a bit on the old cassette. I’m about to order a new one, but I have been thinking about maybe getting to ride in the Alps later this year so maybe it’s a good time to get a bigger cassette. At the moment I’m on 50/34, 12/25, which has got me up everything except Hardknott/Wrynose.

If I went 12/27 would I likely get problems with chain being too short?

It would be 1 link difference with the new cassette in the biggest sprocket.

Unless your current set up is very tight for chain length then it would be fine.

You only drop the gearing about 2 inches though

Depends how short you eet up your new chain with your current cassette 2 teeth should be ok failing that sling another link in. Going up to the 27tooth should enable you to climb pretty much anything esp. if your a spinner.

Do you still have the spare links from the chain? If it’s still new, put a few links back in with a quicklink.