Keep an eye on that ST47 as its a sport servo and I'm not sure if its rated for use in helicopters. I have a couple ST47B's with ball bearings in my 4 Star 40. My JR 537's are rated for .30 size helicopters and have worked well so far in the ZX.

The free movement of the tail rotor drive and linkage is CRITICLE to smooth tail control. You need to disconnect the tail rotor push rod and see how free each component is. I have found that the tail rotor bellcrank that sticks through the housing can drag on the slot that it passes through on the left housing. I usually take a small file and open up the slot until the lever moves freely in the slot. Make sure the bell crank pivot bushing is free from drag at the right tail housing half(I upgraded mine to use the ball bearing pivot). and make sure that your blade grips move freely and that you have greased your trust bearings in the grips. I have also seen some drag on the tail rotor drive shaft where the pitch change bearing slides. I just oiled it real well and worked it back and forth until it burnished itself and smoothed up.

The tail push rod can drag in the plastic housing if its not straight. Especially where the rod moves from the frame to the boom. I usually cut a few pieces of silicon tubing 3/4 of an inch long and slit them so they slip over the plastic housing and then use vinyl tape to hold the silicon tube to the tail boom. One between the frame and boom supports and one between the boom support and tail housing. This removes the tendency for the drive rod to flex when its being pushed aft. I've tried zip ties but its hard not to get them too tight where they bind.

I've gotten my tail rotor linkages to move so smooth that I can just push on the lever at the tail to actuate it back and forth. But remember, I'm using very fast tail servos aka JR 810G's and they dont have alot of torque but lots of speed. They also turn super easy by hand compared to a standard Futaba 148, (my original ZX came with a JR 501 on the rudder and it worked just fine.)

Don't go for a "high torque servo" to solve problems with a stiff tail linkage, instead take a little extra time to clean up the tail issues and you will be rewarded with a tight tail lock and smooth piroette rates.

Looking foward to some awesome hovering stories soon!