The antifreeze/crockpot method is way too much work... storing the crockpot, smelling the stinky heated antifreeze, dealing with antifreeze itself...

Go to an autoparts store and buy a gallon of carb/parts cleaner. I use "GUNK" brand, I'm sure there are others. The gallon should contain a basket to set parts in for dunking. The solution is waterbased, therefore it rinses off nicely with soap and water.

Disassemble the pipe and header assy and put in basket and put in the can overnight. The next day, I remove the pipe and hit it good with some simple green, rinse it good in the sink and the pipe looks brand new. Everything wipes right off. You can do this with the oldest, nastiest pipe... works perfect. And you won't believe how much comes out from the inside of the pipe. When you are done, put the top back on the can and put it on a shelf.