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Installing the tail boom in the right position is critical for the smooth operation of the crown/bevel gear mesh. Push the boom all the way in. Use your finger to push it back out. Then use a hex driver through the hole in the frame to get some leverage and push it out further to attain a good mesh. If the mesh is still tight, remove the shim (by removing the tank and lower bearing block), push the main shaft down, loosen the main shaft collar and move the collar up and re-tighten. The goal is to adjust the space between the crown and bevel gears for a smooth tail operation. Also, we want to ensure the transmission assembly is fully inserted into the tail boom. If it's not, then the bevel gear is free to slip against the crown gear, destroying its teeth.

Proper installation of the boom and setting of bevel/crown gear mesh is as follows:

Fully insert the transmission out assembly into the boom,
Install the boom and boom clamps into the frames (and screws/ nuts),
Push boom forward while pushing bevel gear away from the main shaft and note the output assembly will lock against the front boom lamp,
Tighten boom mount nuts,
Check the tension of the bevel gear to crown gear by rotating the main gear or rotor head. Some tension is desired but not so much that the crown gear is very rough. The crown gear will wear in on the first few flights though so readjust the position of the bevel gear to maintain a mild crown gear tension.

If the shim is already out and you put more down pressure on the main shaft the lower bearing block will prevent the crown gear from moving down unless you also suggest to loosen the 4 screws holding the lower bearing block. Some additional pressure might move the block down. Re-tighten the 4 lower bearing block screws with downward pressure on the main shaft.

Check that the boom pushed in the correct amount so the pinion is directly lined up with the crown gear, if you get bits fraying off the crown gear in use then it's not lined up directly above it.
Check the boom is pushed all the way into both boom clamps in the frames.
The 4 set screws holding the pinion on to the shaft are tight so the pinion can't move on the shaft.
Even with the shim in place, run the model for one or two tanks and then feel if you can rock the pinion backwards or forwards, if you can then the mesh is still too loose and add another shim.
Grab the head and while standing on the skids try pulling and pushing up and down to check for any movement, there obviously should be any if everything is seated correctly and the mast collar is in place correctly.
Both mainshaft bearing blocks fitted the right way up, not even sure if you could put them in the wrong way up but worth checking.

When mounting the boom to the main frame, there are a couple things to keep an eye on. First, make sure your transmission output assembly is inserted all the way into the boom and the boom clamp is against the lip of the output assembly. The slits in the boom clamps will be on the bottom side. Place the bolts in the front end rear clamp to secure the boom while adjusting the placement of the boom. The pinion should be exactly aligned with the crown gear with no overlap in the front or back. You can adjust the mesh by removing/inserting a shim under the crown gear hub. Be sure all four set screws are secured with threadlock and snug.