For caps with a ring around them, I use a round needle file handle (and small diameter tool steel will do) and roll it along each of the brass strips against piece of paper kitchen towel on a hard surface with a fairly firm force but NOT rolling the outer ring. Rolling will stretch the brass slightly, making the funnel cap buckle into a 3D shape - because the perimeter ring remains the same size but the strips inside it try to expand.
Try it on a piece of scrap PE - it's easy - just take your time with it. It doesn't even take long once you have a feel for it.
For caps with a ring around them, I use a round needle file handle (and small diameter tool steel will do) and roll it along each of the brass strips against piece of paper kitchen towel on a hard surface with a fairly firm force but NOT rolling the outer ring. Rolling will stretch the brass slightly, making the funnel cap buckle into a 3D shape - because the perimeter ring remains the same size but the strips inside it try to expand.
Try it on a piece of scrap PE - it's easy - just take your time with it. It doesn't even take long once you have a feel for it. :thumbs_up_1: