They are different per manufacturer, model, etc. An example is my old DT 250 Yamaha is aircooled and has a clearance of .0008 to .0013 thousandths of an inch. My son's "94 yz 250 watercooled is .0016 to .0018 thousandths of an inch. You shouldn't guess at this you need to get a repair manual for that model. There are many factors that would affect it. Such as cast iron liner vs. plated etc.