it will also hurt the parts, the best way is to get some small tubs, fill one half way with ATF and fill one with a mix of petrol+kerosene, dunk the parts in ATF so it can remove some oil and soften it too (ATF is a great cleaner) after a few hours, dunk the parts in the petrol/kero mix.
Steam cleaning is best done on engine parts with a pressure nozzle. It melts away all grime - for that find a service station who has a proper high pressure unit, ask them for a time of when they can start the diesel heater in it, get it really hot and have them hlast away on the parts.