What does a piston ring do on a dirt bike?

Er, the same thing a piston ring does on ANY internal combustion engine: Seals the space between the piston and the cylinder wall, so the force of the gasoline/air mixture igniting pushes the piston down, rather than just "blowing by" the piston and losing lots of power. I don't know what's the configuration of your dirt bike's engine...is it two-stroke (which means you mix oil with the gasoline, like chain saws, weed whackers, many outboard engines) or four-stroke (like modern cars and most motorcycles since the 70's.?) In either case there will be 'rings' around the pistons. Most car engines I've taken apart have a combination of 'compression rings' and an 'oil ring' in grooves on the pistons. The compression rings work as I described. The oil ring scrapes the oil down the cylinder wall so crankcase lubricating oil doesn't end up getting sucked up past the rings and burnt in the combustion chamber. Hope that helps, Paul PS: There were some 2-stroke cars, I remember a guy I knew in high school in the 1960s who had a 3-cylinder 2-stroke Saab. And while 2-stroke engines tend to smoke since they burn up the oil with the gasoline, worn out 4-stroke engines smoke too, as the worn oil rings allow crankcase oil to end up being burned too. PPS: OK, rotary WANKEL or Mazda engines don't have piston rings...but they have tips on their rotors that fulfill the same function, AFAIK. Paul.