Hi,
I want to use the MK Bellcrank on my 90 size pattern can anyone tell what CF diameter i should use and also how
to support it to the fuse ?

Thanks,

I used 3/16" carbon fiber tubes from Central. It has wall thickness around .7mm with 2.5mm rod ends.
You will want to use sponge if you can't mount support for it. I pushed the sponge (to the half way between the elevator servo and bellcrank) thick enough to reach both sides and tall enough to touch the top and bottom of the fuse.
Been flying that setup for more than 7 years without a problem. some trim changes between hot summer and cold weather, but not very big for me.

Again, I use 5/32" CF form Central Hobbies, with NO support, of course that depends on who long is the rod, if you get the CF rod try to bend it form the corners, there is no way gravity will bend those rods in any manouver, if it does not bends in 2x2 YS170 power it should be more than safe on a 90 ship.