After taking my engine apart I have 2 pistons that were gouged when 3 valves were damaged, should I replace only the 2 damaged pistons or do I have to replace all 8?
Were the pistons just gouged or broken? If they only have some material moved around, you and grind off the sharp edges and raised material and still use the pistons. Based on how much material is removed you will need to get all the pistons balanced.
On reply screen go down to > Additional options > Attach Files > Manage Attachments. Also the piston may not look broken, but can be so you need to get them check.
im thinkin that should just replace them its a little safer for the cost. but you can reuse them but, if you plan to do a power adder like boost or spray that will be a really weak link in your bottom end. but if you dont plan to do the power adders and dont have the money in your budget its fine to reuse them