Will fit most 540 / 550 motors. Inner diameter is 35mm by 29mm wide. The heatsink can have a fan mounted onto it. Drill holes have been tapered to fit the following fan measurements: M3 3mm metric thread. To fit a 24mm between centers fan (measured as if measuring one flat side of a square) 4 screw holes. 24mm apart (e.g. top right to top left fixing holes). Images copyrighted by BlackSmith Products Ltd.