Mine did not have that gap when assembled. Mine felt a good bit smoother once assembled, but I did put all fresh bearings in when I built it, so I think that was more of a factor. I put a conventional layshaft in and am running a stock TRX slipper on mine, and it is easily the loudest gearbox I have-- so the plastic must be harder than the usual RPM stuff, because it does nothing whatsoever to dampen noise. I was running Monsterslash with the RPM box and Ultra Slashipede (equipped with an FLM case) side-by-side and the RPM seems about three times as loud.
But the box seems to be very, very well made. All of the screws line up perfectly, everything fits nice and tight. The FLM will always be my favorite, but-- once durability is suitably tested-- the RPM stands to become my most recommended.
There are no atheists in the foxholes / There is no intellect in the air / There are no scientists on the way down / Just a working example of faith versus physics. --El-P, Flyentology