Originally Posted by Dave31
Use a nut splitter or hacksaw and cut the nut off...
Kind or hard to with the cylinder head installed.
Most likely the head nut is a 14mm, for 8mm studs, and a 10mm for 6mm studs (found on some 49cc engines). Take a correct socket and a breaker bar/ratchet (1/2" best but a 3/8" will work, breaker bar is best). Just unscrew the nut..one of two things will happen. 1) the nut will come off, or 2) the nut and stud will come loose. Trash studs and nuts, buy a Metric 8.8 hardware and run a tap to clean threads. Install new hardware and torque to specifications. Like Tom said 6mm are 1.0 thread, 8mm are 1.25 (most common) thread and the uncommon 8mm 1.0 thread.