Then when Rooter's head is large in your inventory screen. Move to the bolt you want and click at the bottom of it and hold it (is how I've done it) - until it turns to a bolt (but leaves Rooter's head large) - and drag it on his nose.
Undoing the bolt is a pain - so a thanks to @Oather for explaining it:
Took a while to figure it out. Select rooter's head as big item on left, then drag the little version of his head onto the big version, and the bolt falls right out.