A really good way to back flush the block is with a power washer or car wash wand right in the drain hole. Blow in with pressure, use garden hose at thermostat hole to flush out crude. Do this a bunch of times till block runs clean. Same applies to radiator only don't use power washer! I have flushed them using radiator cleaner and just hot water. Lay the radiator face up on the floor and fill with hot water and cleaner, let set a while and flush it out.
Joe
