Simple Green, the green stuff and not the yellow cuz it has citris which will attack paint.
Scraping (with soft tool or putty knife) the heavy crud first is best, otherwise you'll simply have the cleanest crud around. Once you only have grime left after crud is scraped, then a liberal application of SG, let it soak in even to the point of drying. Apply a little more and brush what you can with a scrub brush. Then power wash, minding where the spray and grime will splash.
A second series may be required on tough spots.
At least that's how I clean frames and axles and trannys and such.
Cheers... Brian
Cylinder Index = 84
Current Experiment = `57 Tbird, Grand Rapids, Michigan