Dura Ace rear mech on xc mtb

[66 triumph daytona]

Can a Dura Ace M7800 10 speed mech be used on a 9 speed mtb that will be used for light xc?I think that the 7800 mech can be used on both 9 and 10 speed road....

 

[mattr]

Yes.
Pull ratio is the same road 9/10 and MTB 9.
Cage will be slightly narrow, but not enough to effect performance.

 

[66 triumph daytona]

Re:

Cool,what is the max sprocket size with the regular one?Are the long cage versions rarer?

 

[mattr]

http://si.shimano.com/pdfs/si/SI-5V50E-000-ENG.pdf

All the details on there.
Long cage makes no difference to largest sprocket size (27T, but you can get away with more on some frames with long gear hangers) it just means you can use it on a triple. (Capacity and max chainring range/variation increases a fair bit)