The work reported here attempts to: 1) define limits for the design windows for liquid metal breeders and coolants with various structural materials in various tokamak fusion reactors, and 2) quantify the impact of uncertainties in these limits on the design window. MHD pressure drop and heat transfer models are developed and used to quantify the effects of varying several tokamak reactor and blanket design parameters and materials properties. Uncertainties in the present pressure drop equations and calculational methods are also considered. Calculations are used to evaluate the impact of the coolant inlet temperature on the thermal cycle efficiency. An evaluation of the limits of uncertainty gives results ranging from a promising blanket candidate to a severely restricted design window, that would probably exclude self-cooled liquid metal blankets for large tokamaks from consideration. The major uncertainties in the design window result from the current lack of understanding of pressure drop and heat transfer in strong magnetic fields.