I don’t have a magical system, I mainly use Seafoam/Deep Creep, oven cleaner, etc… and an appropriate pad and elbow grease.

A local engine rebuilder bored a block for me once. I happened to have the pistons in the box with the block. They dunked it in their concoction, and they came out like brand new and polished! They wouldn’t tell me what they use….trade secret apparently.