After removing the coolant petcock from the driver's side bottom back of the block, I found the coolant channel was clogged in both directions on my 51 Chevy 3100 with a 61 235 in it.
After digging out all of the coagulated sediment that I could with wire, I took an old motorcycle clutch cable and cut the outer jacket 6-8 inches shorter than the inside cable and hooked the drill to one end.
Used the jacket to guide the cable in and around the corners at the back off the petcock hole , then used the variable speed drill to turn and push the inner cable in and out and routed out all of the gunk that was in there. Put a hose in the radiator and once it broke through kept running the cable back and forth, in and out, until the water came out clean.
