Four letter words--Change 0 to 1 letter to make a new word

pass

sips

saps

hasp

rasp

part

port

fort

frat

frot

NSFW: [spoiler]https://en.wikipedia.org/wiki/Frot[/spoiler]

TORX (a kind of wrench bit, you know)

rots

Rota

star

stab

bars

bats

stub

must

most