Abstract: A spray nozzle for spray gun for forming a polyurethane layer on a surface by spraying the reaction components for obtaining this polyurethane. A spray piece (21) with a funnel-shaped cavity (27) is connected to a supply channel (19) of the reaction components and flows out in a cylindrical channel (29) which connects this cavity (27) to a spray opening 22. A core (17) is detachably placed in the spray piece (21) and allows the reaction components and/or the already formed polyurethane to be fed according to a screw or whirling movement through the spray opening. The core has a conical part which rests against an inner conical wall of the cavity. Formed in the conical core part are spiraled grooves (23) which impart a screw or whirling motion to the reaction components and which are in communication with respective core borings (40) which open into the supply channel (19) at a common point.