Beware with this kind of pinions, normally the motors use a 3.17 shaft, so this gear is made for specific models only !!!
please be sure your shaft id 5mm.
If you buy a 3.17mm and it dont enter correctly, you can heat the pinion to expand a little bit the hole. That's why they use that milimetrical measures.
You have to choose a Pinion Gear with the same pitch as your Spur gear.Maybe if you look on your Spur will tell you the Pitch (or just "P") number.So if it sais something different than the "32 P" then u should search a pinion with the same Pitch as your spur or a spur with the same pitch as the pinion!