This example shows a worn piston ring seal groove – in the image, you can see that the outer edge of the piston ring seal groove is sloping outwards, when it should be straight.
This is fairly common, and has a number of causes. Sometimes, it’s a simple case of wear and tear caused over the lifetime of the turbo, but it can also be caused by excessive wear in the thrust or journal bearings.
In addition, this kind of wear can also happen when carbonised oil builds up in the seal area, which then rubs at the groove during operation, wearing it away. Good oil management (checking and changing it regularly), using the right specification oil, and regularly changing your oil filter will help to prevent this.
Once a piston ring seal has become worn in this way, the best course of action is to replace it.