valjak

1. roller
   Zidove sam okrečio valjkom za dva sata. — I painted the walls with a roller in two hours.
   Na ulici radi valjak, pa je put zatvoren. — A road roller is working on the street, so the road is closed.
2. (in geometry) cylinder
   Izračunajte zapreminu valjka čiji je poluprečnik pet centimetara. — Calculate the volume of a cylinder with a radius of five centimetres.

An engine cylinder is usually cilindar, not valjak.