Uhhh, they have a CONVEX surface not concave. If it's concave, you are in the poop.
If you google lifter resurfacing machines you can see what they look like. Been around forever.
As Krooser said they used to make attachments for valve grinders to resurface lifters.
The real issue is lifters have gotten cheaper so the unit price is through the floor. If you have the machine and it's paid for you can make some money. To go and buy a machine isn't cost effective.
Plus, unless I know the history of the lifter, I never resurface a hydraulic lifter. The hydraulics wear out.
