[cnavarro]

Oh, I didn't realize you were talking about the CE/sealing ring/gasket groove on the top of the cylinder. I thought you were referring to the ring lands on the pistons. I've never had to do what you're doing. I would venture to say that there's no real easy way to clean those. Lots of patience is all I can offer to you. You have the right idea to use something plastic as to not scratch or damage the cylinder. Maybe someone with more experience re-using Mahle cylinders can chime in.