I have fitted a 10 speed shimano cassette,chain and shifters to an all shimano transmission ,all the other transmission bits are in good condition rear mech,cables,drop out etc,b tension is set up fine ,should set up to shift nicely but is proving a right git.I assumed the rear 9 and 10 speed mech`s still have the same parallelogram and pull the same amount of cable per shift, or is their another reason why a 9 speed rear mech wont work with 10 speed ?

Shared wisdom would be much appreciated

Different cable pull ratio.You need a 10sp mech.

Aye, 10 speed needs a 10 speed specific mech I’m afraid.

Shimano 8, 9 and 10 road stuff and 8 and 9 mtb stuff are all cross compatible. Shimano 10 mtb is totally different sadly so youll need a new mech.

thanks for the help lads and for letting me know, glad its the set up and not me, new rear mech it is then.hay ho.