A lock includes a housing, a cylindrical core body and a tumbler unit. The core body is rotatably mounted in the housing, and includes a core axle, and a core tube sleeved on the core axle. The core axle has a helical keyway. The tumbler unit includes a housing pin hole formed in the housing, a tube through hole formed through the core tube, a pin assembly movably mounted in the housing pin hole and the tube through hole, a hole-defining surface defining the tube through hole and having a shoulder surface portion, and a resilient member disposed in the housing and resiliently biasing the pin assembly to abut against the shoulder surface portion and extend into the helical keyway.