As Bob has pointed out, it can depend on what your idea of clean is, and the engine. I have successfully done the quick dunk and run on a couple of plain bearing engines such as Foxes, McCoys, and OS FP engines with no problems, Just drain them as best you can (pop off the back plate if need be), then flush with some old fuel, put in a known good, used plug, and burp them several times to blow it everything. I short two or three minute bench run will clear them out also. I have done one of my ST.51's that way, but it was in regular use, wasn't stuck, just wanted to get the head and crank case clean. I flushed it out, burped it several times, installed in the airplane and went flying. Would I do it to a high end PA, Ro-Jett or expensive Russian engine? Probably not.
Type at you later,
Dan McEntee