GY6 performance cylinder head for 63mm bore
If you are going with a 180cc (63mm bore) on your GY6 you will need this head. This has the down cut to match the 63mm bore. Not to mention the porting is much better. This has the common 76mm stud spacing (GY6 A).
By the way, this is the good stuff! No eBay/Amazon crap :-)