Riles

I thought I purchased what was a Simano XT long cage 9 spd RD... the model name is M751 & it is a low/normal (rapid rise) type.

when I put it on the bike, it only travels as far as the 7th cog & it is nothing to do with cable tension.

I put my old XT RD back on and that is fine (but short cage) so I assumed the new one must be a 7 speed RD - hwvr, when I look on the Shimano website it seems to indicate that the M751 is only a 9 speed ?

So I guess my question is - how can I tell what number of cogs it is set up for because it doesn't say on it and was I right in thinking that due to the fact in only traveled out the 7th cog even when I pulled the cable hard with my hands that it must be a 7 speed ?

TallRider

Nearly all shimano rear deraillers since the beginning of indexed shifting are compatible with each other (with the exception of pre-9-speed Dura-Ace). Basically, they all have the same cable pull ratio.

My guess is that the limit screw on your rear derailler is simply screwed in too far. Back it out and give hte derailler more room to move.