Soak the head and manifold in muriatic acid diluted at least 50% with water. Available at Home Depot or Lowes, usually in the paint
section. Will foam sizzle and bubble as it dissolves the salt
and scale blocking the passages. Had to do several times on a couple of MD7As I rebuilt. Be sure to neutralize the acid with a thorough water wash down.
When you put it all back together, hook up a 12 volt pump
and circulate a fresh batch of the solution through the block for an hour or two. This will clean the passages in the block.
Hook the intake side of the pump
to a hose that goes to the bucket. The output side goes to the raw water
intake on the engine (after the raw water
pump). Then run a hose from the raw water output on the exhaust manifold back to the bucket to complete the circuit. Don't forget to remove the thermostat for this process. When done, flush well with fresh water and replace the thermostat. I do this once a year with my raw water cooled Volvos and have not had an over heating
problem since adding this to my maintenance