My RC550s don't fit very well either but with some clever use of RTV silicone, they will not leak and will work okay. Although I would have liked it better if the injector nozzle was closer to the opening on the inside of the manifold. Now it has to spray thru this long tunnel which makes me wonder if the spray pattern is degraded any. The only way to remedy this is to have machine shop bore it out just right. I might do that if I can find a cheap used manifold. How much do they sell for usually?