This is a question that comes up a lot. There are a couple of different methods that are reputed to work well. Personally I have only used the boiling it in anti-freeze method. It worked very well but has since been brought to my attention that it is not very good for the environment, & it can also be a fire hazard if you aren't carefull. I did it in the back yard on the side burner of my gas Bar-B-Q in an old coffee can. It was a 30yr. old+ OS .60 engine that was completely froze up, couldn't turn it over at all. It came loose & in a fairly short time came out looking like it was brand new inside & out. I have also been told since then that it may eat up the rubber o-rings. So far everything still looks like new but we will see.

piper chuck has a method using rubbing alcohol & the oven I believe, that sounded much safer & cleaner to me. I will let him tell it though as I don't want to mis-quote him.