RE: trex 450, hbk2 interchangeability ?
I've been using the same Trex HS1003 spindle in my stock King head since October through 3 crashes. No bend. I only use the one O-ring per side with one spacer per side. Fits and flies like it's supposed to be there. There may be 1/2mm of in/out play if you yank the grips hard enough, but my blades track like a knife and I've had no flight issues whatsoever.
I may remove the spacers and try the doubled O-ring thing, though.