Is it bad to clean NES games with alcohol?
Pure alcohol is best, but anything above 50% alcohol will work. Ignore the warning on the back of Nintendo cartridges that tell you not to use alcohol. This was a marketing ploy used by Nintendo to sell cleaning kits. This process takes time, and may take more than one cleaning.
What is best to clean NES games?
Rubbing alcohol.
The higher the concentration of alcohol the better—you need this liquid to clean up the contacts after you scrub them, but you don’t want a lot of water in it because it can rust and corrode the pins. Stay away from anything 50 percent or lower. A gentle, lint-free cloth.