RE: Black piston?
Hi!
This is what a typically castor-oil-run-engine looks like after a year or more running.
Nothing to worry about really but must ask...Why did you take it apart? Did it run bad or what?
If you had run it on the modern synthetic oils that are available now you would not have any carbon build up at all. In Europe it is very common to use just 10-15% all synthetic oil. Castor oil is very seldom used over here.