When I strip a TR gear, this is the way I like to reset the gear lash:
First, remove the tail boom from the model (leave the front boom clamps tight on the tail boom).
Next, remove the front Tail Transition Support on the Front Tail Boom Clamp.
Now, check to see if there is any end to end play on the tail rotor output shaft in the TR gearbox. If so, use shims or slightly bend the stock spacer to eliminate the end play.
Now, check to see how much clearance is in the bevel gears in the tail rotor gear box. If it seems like too much, first remove the 2X10mm allen bolt that pins the TR gear box to the bottom of the tail boom (step 5-2) and then loosen the 2X6mm allen bolt that clamps the tail rotor gear box to the tail boom. Now, slide the tail rotor gear box in a little on the tail boom to tighten up the gear lash then tighten the 2X6mm bolt on the TR gearbox clamp.
Then, I like to spin the tail rotor using my finger on the front bevel gear on the torque tube. It should be very smooth without any binding and have a little gear lash (plastic gears don't wear in). When it's smooth reinstall the 2X10mm allen bolt that pins the TR gearbox to the tail boom.
Now, reinstall the Tail Transition assembly. Check the clearance between these bevel gears. If it seems a little loose, loosen the screw on the Front Tail Boom Clamp and slide the Tail Boom a little further inward to reduce the clearance.
Now, spin the TR assembly again using your finger on the 24T Flat Gear and all should be smooth (no irregular drag) and you should have a little gear lash.
Now, reinstall the tail boom and tighten up all bolts. Check to see if there is very much clearance between the 24T Flat Gear and the 100T Tail Drive Gear. If there is too much clearance, just loosen the clamping 2X6mm screw on the rear Tail Boom Clamp (do not loosen the clamping 2X6mm screw on the front Tail Boom Clamp or you will change the gear lash on the front bevel gears) and then loosen the (8) 2X6mm screws that hold the tail boom clamps in the side frames and push the tail boom assembly a little further inward. It doesn't appear to be critical to have a very tight clearance between the 24T flat gear and the 100T tail drive gear.
Now, (with the main rotor blades off) spin the main rotor head by the flybar to see if if you hear or feel any binding. Once everything's smooth tighten all screws.
The tail drive system should run smooth.... And while holding the 100T Tail Drive Gear, the tip of my TR blades move back and forth about 8mm (accumulated gear lash).
Keep up your rotor speed !