Direct from the product description on treats:
"note: this kit needs a mm or two of spacing to allow the piston to clear the head. you'll either need to get your head machined out, cut it by hand or run one of these handy mlm spacers to get things spaced nicely. if you're a super pro, you'll use the mlm spacer, then get your head cut to account for the additional deck height from spacing it. "