The 1st part of te break in is the most important one. On a piston it surface there is a very small screw thread like structure made by the lathe making the piston.




The top points of the piston structure are basically very fragile and need to get flat spots w/o burning and/or breaking them. When that is done the piston can stand more forces w/o getting damaged. The oilbath with 100% lubrication and no real expansion heat will give the piston a much better polished surface.