Rear derailleur query

Muckers

I've a Shimano Tiagra 4500 triple rear mech on my winter bike, which i'm getting ready for those upcoming wet and cold spins. I've upgraded the cassette from a 12-25 to a 12-27, but I cannot shift it into the smallest rear cog. The B tension screw makes no difference to the problem. Instead of shifting up onto the smallest rear cog, it's shifting into the side of it. Turning the low limit screw doesn't help either. A mate of mine suggested that the cage may be too short to accomodate the larger replacement cassette.
Any opinions and if I need to replace the rd, what model number should I go for. Thanks

doozerie

Check the Shimano website (www.shimano.com), it'll have the specs of that derailleur in there which will tell you whether it can handle a 27 rear cog (in combination with whatever chainrings you have on the front).

The issue could be your chain length too, you might need a longer chain to work with that larger cog, although a jump of only 2 teeth over your previous inner cog is small enough that I would have thought the old chain might work - it all depends on whether your chain length was already at its limit with the 25 inner cog.

A more obvious one, but worth checking, is whether your gear shifter is taking up the slack on the gear cable. To rule that out, pull on the rear gear cable with your hand to see if that allows it to shift.

Having said all of that though, if the upper jockey wheel is hitting the the side of the 27 cog, then it does sound like the B screw on the derailleur could be the real issue (assuming though that the derailleur is spec'ed for a 27).