nitro fuel. I say nitro fuel because it wont harm the engine at all because its made for it. Im sure other stuff will work but the risk is bar none with fuel and the fuel also eats chemicals and gummy fuel. What i normally do is get a lil cup or old plastic bowl you dont use any more and i poor some fuel in it and lay my parts in it, letting them submerge for a while. After they sit in there just check on them and wipe them off. Inspect the piston/sleeve for scarring from possible dirt or grit and see if it causes resistance when putting the piston in the sleeve. I always say never use wd 40 or any thing on ur crank because if it has epoxy the wd40 will cause the empoxy to expand damaging the crank, im not sure what other brands will do this so just be careful with what you decide to use. Also if the motor still has life and you wont it to last after all this has been done i would say sealt the carb and backplate with engine sealent. Lucky 7 rc makes some and you can find others to.