Before installing the pistons in a newly honed cast iron block, you should clean the bores with:

Cleaning the bores with soap and warm water is the most appropriate choice for preparing the surface before installing pistons in a newly honed cast iron block. This method effectively removes any residual honing materials, oil, dirt, and other contaminants that may be present, ensuring a clean surface for piston installation.

Using soap and warm water helps to dissolve grease and oils thoroughly, while warm water enhances the cleaning process by making it easier to remove stubborn residues. It also ensures that any particles dislodged during the cleaning can be washed away effectively.

While other methods like using acetone or denatured alcohol can clean surfaces, they may not be as effective in removing all the contaminants without leaving residual solvents that could interfere with proper lubrication or piston function. Compressed air, on the other hand, can blow away dust or loose particles but does not address oils, residue, or debris left in the bores, making it insufficient as a standalone cleaning method prior to installation.