Originally Posted by
FBinNY
This may be off base, but worth checking to confirm anyway.
Like with all derailleur systems, the indexing in Di2 is built into the controller, not the RD itself. So while an 11s RD may have a slightly narrower cage, there's a decent chance that it will act as a 10s RD when mated with a 10s control. It's the control which tells the RD to move over by X amount, and the RD responds accordingly. Of course, there may be a feedback loop, or other factor which negates this, but it's worth a shot to call someone at Shimano tech support, or speak to a dealer with intimate knowledge of Di2 before tearing your hair out.
In any case, it shouldn't be impossible to find a NOS derailleur someplace, possibly at a lower cost than current stock.
This is incorrect. Its the RD that determines what speed the system is. The 6870 RD is fixed in 11 speed. The shifter buttons just send a signal or 'instruction' to the derailleurs to make the shift. The distance that the cage moves is pre-programmed into the derailleurs.
if you are on the old firmware before v2.6.0 and you plug the 6870 RD in it will turn the system into 11spd. and will work with the 6770 FD.
If you install the latest firmware v2.6.0 or after , then you will need to buy the 6870 FD. Because the 6770 FD will no longer work.
You can no longer mix 11spd RD's with 10spd FD's, or 10spd RD's with 11spd FD's after installing the v2.6.0 firmware.
There has never been an option to use the 6870,9070 RD's with 10 speed spacing.
All compatibility info is on this page
http://carltonbale.com/shimano-di2-e...-need-to-know/
chrisdr2015, You only have two options. Find a used 6770 RD on ebay,classifieds etc or find a NOS RD from a retailer/distributer.