The best product to use as a piston soak is Berryman B-12 Chemtool Fuel Injector Cleaner (part #s 0116 and 0121).

To perform the soak, remove the spark plugs, add to the COLD combustion chambers enough B-12 Chemtool to cover the entire piston crown and rings, and put the spark plugs back in a couple threads in order to prevent evaporation of the solvents.  The process will benefit from a little agitation, too, so if you can safely rock the vehicle from side to side occasionally, that will improve your results.

Obviously, you’ll want to be careful doing this (or avoid it all together) if the car is on a jack or jack stands.  You can also carefully bubble in a little air either from a compressor or an aerosol duster.

Regarding duration, we recommend soaking the pistons for at least 1-2 hours apiece.  However, if leak-down is minimal, you can soak for much longer.  Some users prefer to soak overnight, that’s completely acceptable.

Once you’ve decided to end the soak, the cylinder will need to be evacuated of the cleaner/carbon mixture.  You can do this any number of ways including with a vacuum pump or even a turkey baster (which should not be used again in the kitchen).  Note that B-12 Chemtool will damage most automotive paint and clear coats, so be cautious around such finishes.