Depends on the machine, in our case it would be a cylinder machined to accept a larger sleeve that increases the usable bore size, along with a piston to match the bore size. this increases the cc's of the machine. ie a 250 to a 285 or 310. carl (iceracer) and gillio have more information and experience on the subject. I will be having a big bore built in the next year or so for MX.