soloboss

Sorry the information was ambiguous ( i love that word).
You want 3 3/8" of exposed metal ON EACH SIDE of the plastic part of the flybar. You are absolutely correct that a total length of 3 3/8" is too short for a flybar. You could go that short if it weighed enough, I suppose, but it would be as heavy as the battery pack!!

Remember that the control from the flybar is a result of the spinning mass. You have a choice of using a long flybar with light weight or a short flybar with heavier weight.

I had a brainstorm once and used a flybar with NO weight. I cut the hook off the end of the metal and put a long piece of CF tube over the metal. The CF tube we . . . wow, I don't recall exactly, but I'm thinking it was 8" long measured from the plastic hub to the end - oneach side. That flybar was a bit longer than the blades. I think it would have worked, but the wind resistance from a long flybar like that was too much. It slowed the upper rotor response.

That brings out another issue. If you get the flybar to spin more easily (some combination of shorter / lighter) the upper rotor assembly (blades and flybar) will accelerate faster. Ideally the upper rotor assembly and the lower rotor assembly will accelerate at the same rate so the tail stays in alignment when you hit the power.

Keep playing with the flybar and we'll see if you come up with something we missed.

EDIT: I forgot to answer our questions about the boom. That's a BoomTownHobbies boom and skids. They are about as close to unbreakable as it gets.
Soloboss