Mike wrote:
whats involved in replacing it? i know i need a 27mm socket....
You'll need a socket like you said.
Liberal doses of WD40 or similar.
A piece of angle iron about 70cm with 2 holes drilled and bolted to the pinion to stop it turning while you undo the nut.
A 2 or 3 jawed bearing puller of rather beefy dimensions is also handy. Those pinion flanges can be very determined to stay put!
Jas.