I use rubbing alcohol. I've heard others suggest brake cleaner, boiling in antifreeze, and many others. They all probably work the same. I figure if they use rubbing alcohol in hospitals and doctor's offices, it can't be too hazardous. Soak the engine for a little while and use toothbrush, pipe cleaners, etc to get into small areas. Once done, heat the engine a bit in the oven to evaporate the alcohol and then oil it up.