Put a new set of jackey wheels on my rear mech earlier. Basically it's thrown the indexing way off e.g. 2 full cogs on the rear cassette. Which has never happened to me before. Now i've no problem adjusting it to get it back indexing grand again. However cause it's never happened like this to me before it's got me questioning whether i've made a mistake on the install. So anyone else had this problem? Is it common enough & am i just over reacting?

floating jockey wheel at the top ??

Originals were shimano floating jockey wheels new ones are FSA as far as I know they're not floating.

Did you change the wheels with the mech still on the hanger? If not could have altered in another way rather than it being the jockey wheels. If not could the jockey wheels possible be handed, odd one that, 2 cogs is quite a distance.

Float on jockey wheels = 0.8mm, 2 full cogs = about 6mm, i'm thinking you must have done something else too.What type of mech? Have you actually been able to re-index the gears? Did you disconnect the cable, and route it the wrong side of the clamp bolt maybe? Have you accidentally bent the hanger whilst working on it?

Right to clarify the mech is an XTR M971 which i got off a mate when he upgraded to the shadow version. He didn't use it all that much prior to upgrading. I've used it fairly heavily over the last 9 months. So figured it was time to change the jockey wheels whilst changing the chain as the indexing was starting to get quite sloppy. Bought a set of FSA replacement jockey wheels like these http://road.fullspeedahead.com/fly.aspx?layout=product&taxid=115 Installed them as far as i know correctly. Took the rear wheel out, chain was off cause i was replacing it.

Haven't tried re-indexing them any more than using the micro adjust on the shifters (there isn't one on the mech). Was just concerned as in the past any time I've replaced jockey wheels they've required nothing more than micro tuning to get perfect not having to actually readjust the whole cable at the derailleur. If that makes sense.

OK, try checking your cables, you may have dislodged one from the stop in the frame/mech/shifter, this will put your shifting out by about two stops.

Coatsey good call. Will check that in the morning.