as for cleaning the black only on top of the piston use toothpaste dab libererally on let sit for a bit then polish off in a circular motion

I use a water based de-greaser in my heated ultrasonic.... Gets the engine sparkling clean !

Fuel works good. I have a liter jug of fuel that I use specifically for engine cleaning (20% nitro, 16% oil) The nitro will free all the gummed up parts and old/built up oil and carbon in the engine and the oil in the fuel will coat all the parts and ensure no rust happens. You can get in there with some q-tips and pipe cleaners to get the really gummed up stuff off. Just dont do any scraping or rubbing to the piston skirt or the inner part of the sleeve.

The part in bold is true, if the oil is used on it's own. Otherwise we would not need to remove as much fuel as possible after running, or before storage - because the the nitro is 'Hydroscopic' (correct spelling or word?) meaning it attracts moisture from the air?

Methanol is hygroscopic.

Both, methanol and nitro methane are highly hygroscopic...

Perhaps you did not understand....

Even the most synthetic oils are a bit hygroscopic

Thanks guys, I thought it looked a little wrong :p

The black stuff is from not enough oil lube. and high nitro content. Add more oil lube and you will be fine.. All my engines are clean from using the correct amount of oil lube 18%. You can clean it off with oven cleaner and a scotch brite pad.

The black stuff is for a part a burn of the dirt in the castor oil because castor oil is hard to get pure and for a part the burn of the oil itself.

Has nothing to do with a low oil or an high nitro content to do. Well..... for a part. If you run a high compression with a matching cold plugthen there is a slight cleaner burn.