The bulk update request #11536 is pending approval.

create alias asscheek (0) -> butt (1134461)
create alias asscheeks (0) -> butt (1134461)
create alias asscrack (0) -> butt (1134461)
create alias ass_crack (1) -> butt (1134461)

Reason: Among the first two, the singular tag is rare, but the plural tag is fairly common:
https://e621.net/post_versions?commit=Search&search%5Btags_added%5D=asscheek
https://e621.net/post_versions?commit=Search&search%5Btags_added%5D=asscheeks

butt already has the no-space bootycheek, bootycheeks, buttcheek, and buttcheeks aliases, and these follow that pattern.

Among the last two, both tags are surprisingly common:
https://e621.net/post_versions?commit=Search&search%5Btags_added%5D=asscrack
https://e621.net/post_versions?commit=Search&search%5Btags_added%5D=ass_crack

butt already has the aliases buttcrack and butt_crack, and these follow that pattern.