There are two important factors on a spur(pinion gear) it has to has to have the same PITCH (tooth spacing as the rack and the same pressure angle. I don't know about China but there are two predominate pressure angles :20 deg and 14 1/2 deg. Once you know the pitch you get the diamter by counting the teeth The DP is the diamter of the pinion at the root of the teeth (somewhat less than the overall diameter. The pitch is the DP times PI divided by the number of teeth. YOu can measure it on the rack by using the valleys per MM. So a 27mm diamter is an 84.9 mm circumference so to pitch is somewhere around 4/25mm Seems kinda weird numbers