It is important to have some sort of a a restrictor in the inlet of the heat core. Ford has a restrictor in the heater core inlet hose. From 1990-2002, GM uses the plastic clip (clip only-GM 15739936 or Dorman 800-405) ) inside the quick disconnect fitting (fitting with a clip - GM 12543621 or Dorman 800-401) as a coolant restrictor. If you must remove the heater hose for any reason and damage the plastic clip, you can purchase new plastic clips separately so you do not have to replace the entire fitting and risk ruining the threads in the aluminum manifold.
Without restriction, the coolant pressure can balloon the heater core and cause the heater core to leak. The coolant pressure is especially high above 4000 RPM. I have a thick wall intake manifold nipple fitting in my parts bin. It has a 1/2" passage for the heater core inlet coolant. I purchased the mipple fitting at a local Chevrolet dealership but I do not recall what the part number was. It could be used to replace the quick connect fitting.
Many years ago, I pushed a old 1/2" drive socket in the 5/8" heater core inlet hose of my '55 Chevy. That would be called "redneck" engineering today.
Last edited by MouseFink; 03-19-2013 at 05:31 AM.