Gear Notes: "Modulus" is the metric systems way of expressing gear tooth pitch or size. In English or SAE gears teeth are counted per inch. So, a straight gear (like the rack in a rack and pinion setup) rated at 10tpi, would have 10 Teeth Per Inch. Modulus is used in the Metric system and it's an expression of how large each tooth is. Little different system but one thing is held true. A 10tpi and 14tpi SAE gears will not mesh with each other, the TPI numbers must match. Same for Metrics, the "Modulus" must be the same to mesh, a .4mod and .3mod gear will not work together. For GWS gears, all you really need to know is what power system series you have (LPS,IPS,EPS,EPS400 etc..) and the gear letter. On your existing spur gear has a letter is molded in. If you need to replace a "C" gear in your EPS350, forget about how many teeth it has, just order the "C" pinion or spur that you need in the EPS list. Pinions are the small metal gears on the motor. Spurs are the large plastic gears that are on your prop shaft.
A special note to gear purchasers for non-model aircraft purposes. We get many inquiry's asking if these gears will fit a Child's X-G-Wiz-Go toy or will repair a home kitchen device or etc. We are model aircraft experts, if it's a model aircraft gearbox, we can help figure out the part on the phone if your stuck. If it's a toy or special project outside the hobby, I can't begin to guess if our gear will mesh with your gear in your device. I can't tell from a photo what pitch your gear is. If your trying to fix a special item, my best suggestion is to purchase all the gears that look similar and have a go at it. Most gears are a few dollars or less.