I put a new plastic tail unit on my 450 and it does fine for the 1st 30 seconds or so but then the belt goes slack. No matter how tight I make the screws in the tail unit, it seems to not be enough. Any thoughts?

you tighten the screws on the frame? If so you probably need to replace your tail boom

Try a wrap or two of Teflon tape on the end of the boom to take up the slack a bit. Sounds like there is a tolerance variance there where the tail rotor unit doesn't fit on the boom as tightly as it should.

I'll re-ck the frame screws, but I'm pretty sure they're tight. I'll also give the tape a try on the front end of the tail unit. Thanks guys!

If you don't want to replace the tail you can get by using a drop or two of super glue on the tube before you slide the tail on. Make sure you get it verticle and the belt tension set quick before it sets up. When you need to remove it, use a heat gun on it but not too much and twist. I had the same problem with the boom coming out of the forward plastic holder on my 600 no matter how hard I tightened the 3 bolts when doing fast piros. On a 600 with a torque tube that is not a good thing as the torque rod becomes disengaged, unlike a belt which would tighten as the tube slips out. Yep that made for an exciting piro crash landing.