Recommendations for Ultrasonic Cleaner Fluid

We use one in our transmission rebuilding shop to clean the gears and related parts. Mostly steel and cast iron pieces, so I can’t say how any zinc or pot metal parts would do. We’ve used Simple Green and other similar soap based cleaning fluids......mixed about 50/50 with clean water. Works great but it helps extend the life of the solution by precleaning the parts if they’re particularly greasy/dirty. At some point we have to drain the stuff and refill with fresh solution but its a timesaver. Pop the parts in for 6-8 minutes and work on something else for awhile.....come back and flip the parts over after a quick wipe, then run for another 5-7 minutes. The heat is also important for the cleaning process.