Im trying to order some new tires for my little brothers blaster. But i dont know what size it needs. Stock rims all around. Any of you guys have an idea?

i really don't think it is 21x7x10 for both front on rear.

 

The idea is to get the rim size and get the tires you WANT,.... if you get a smaller size than the original, it will give you a significant torque gain, but with a top end loss on speed.

Factory sizes are 21 x 7-10 front 21 x 10-8 rears

This is right off the Yamaha website archives