For cleaning grease and grime off of small parts I use an ultrasonic bath with a strong detergent. Of course, then you have to dry and oil the parts immediately -- you cannot let it sit overnight because the nice clean parts will flash rust.

For cleaning really dirty larger pieces I use a parts cleaner with Varasol, followed by a hot water pressure wash, and lots of elbow grease. Again, beware of rust.

For cleaning bearings I use kerosene and chase it with brake cleaner.

For hot tanking engines or radiators, I let the shops deal with it. I would not want the problem of disposing of the chemical crud that results from using a hot tank.
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Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL