Just to add to this discussion. If you run the Carbon Extreme dampner kit, the B setup flies most like stock. The stock teeter rubbers really fly smooth, have great stability in FFF but get rather loose over the course of time. I did not use the provided drill bit because it would have drilled the clearance hole a little bigger than I wanted so I used a number drill that is just over the 4mm OD of the SHCS. In addition, use a bottle of white out to mark the screws and remove them prior to taking a Dremel Cutoff wheel to trim them. The cutting process heats up the screw and there is really no need to heat up the Carbon Extreme plastic parts.
Another tip is when you build the tail pulley assy. Check to make sure that the endplay on the shaft is minimal. After a gallon of fuel I had to shim my tail rotor output shaft with one 5mm ID shim that I made from .005" shim stock. If after a little flight time you notice the tail vibrating up and down, this is usually traced to a slightly loose belt.
Dial the fan to .0015" or less. After you get the fan hub dialed in, if the clutch shaft runs out even if you flip it 180 degrees just take an X-Acto knife and remove material where the Clutch boss fits into the counterbore of the fan hub. This will give you some clearance so that the clutch can be moved around to get it dialed in as close as possible. Be sure to check the runout at the base of the start shaft and the very end. The WC Fan hub is a nice tight fit on the crankshaft of the 91 C-Spec. Use a crank locking tool and get the nut tight. Don't forget the key in the crankshaft and the washer.
Once you have the mainshaft installed, mark the mainshaft with a magic marker, install the collar and tighten the set screws to mark the shaft. Once the shaft is marked take a Dremel and grind a flat at each set screw location. If you don't, the mainshaft will slip, guaranteed.
Trial fit the blade grips and only use the shims that you need. There are 4 in the kit and you may not need any or you may need all 4. It will depend on the length of the hub and the traverse shaft. If you want, you may use a set of SHCS to replace the button heads that come in the kit. You can get these new screws a lot tighter and I typically don't use loctite as I haven't found it necessary. This is a personal decision and if you aren't comfortable in your building skills use loctite. I don't want to get into a pissing contest about this little detail. I've been building hells since before the traverse shaft machines first appeared on the market in the mid 80's with the Heim and Schluter machines and I haven't had a single failure.
Do a search for Augusto's auto hub mod. I can't speak for its effectiveness but it can't hurt to do if it adds life to the auto bearing. I skimmed .002" out of my hub to give the one way some clearance and this seems to do the trick.
I run the middle mobiity setting on the flybar. If you run the high mobility settings be prepared for some fast cyclic rates. I also run only one weight in the flybar paddles. Actually if you didn't run any weight it probably would still fly smoothly.
If your muffler runs really close to the SHCS on the side of the frames you can swap out for a flat head screw and a trim ring to get some extra clearance. I had to do that on a standard Freya because of the Funtech Muffler I use.
The shroud can be a PIA to do if you have never done a shroud before. On my X-Spec I cut the shrould for 1mm clearance on the top of the Viper head and I left the sides of the shroud intact for some extra material. Be sure to cut off the mounting posts per the instructions, otherwise the shroud won't bolt up. The standard Freya shroud has much more clearance than the X-Spec and I haven't had any cooling issues so far with the standard Freya. A drum sander on a Dremel is a good way to smooth up the cuts on the shroud.
Depending on the gear ratio that you go with you may want to change the pinion on the clutch bell. It isn't hard to get the pinion off if you know how. It is loctited from the factory. Mount the pinion in a vise VERY LIGHTLY so it won't turn, preferably a vise with a set of smooth jaws. You don't want to actually clamp the pinion just keep it from turning. Take a rubber bungy and wrap it around the clutch bell and turn off the bell by hand. The threads are Right Hand so keep that in mind. The bungy will give you some leverage. A drop of blue loctite will secure the pinion back on the bell. Don't forget the o-ring and washer when you put the bell back on the clutch.