Most yoyos commonly use 8mm; however, some take 10-12mm with the latter being rare nowadays. Look at the specs of the yoyo and hopefully it will tell you. It's important for one side of the axle to have a hole where you can tighten it. Obviously, yoyo stores carry them too online. Check this out https://www.amazon.com/MAGICYOYO-Responsive-YoYo-Bearing-Removal/dp/B07MVRSC55/ref=sr\_1\_7?crid=1FBTK1FXCWY6E&keywords=MAGICYOYO+KIT+YOYO&qid=1673337636&sprefix=magicyoyo+kit+yoyo%2Caps%2C103&sr=8-7