My mechanical seal failed recently and I replaced it with identical parts. However when I thread the impeller back on the motor shaft, I run out of threads before the seal becomes preloaded by the seal spring. I've triple check that the seal is the same dimensions as the original. It appears the spring cup needs to be shimmed out a little. The original was not shimmed but it did have a very large bead of RTV around the perimeter and possibly under the lip that may have acted as a shim. Is this common? What shim thickness is required to preload the seal without overloading it? Any other ideas? (I verified its not working by putting it all back together correctly and opening the inlet valve...water poured out the pump backing plate.)