mod 1 pinion . bore ????

[notatoy]

hi guys i need some help i am doing a nitro conversion , now the motor i have has a 3.17 mm shaft . i know i need a mod 1 pinion question is these pinions are 5mm bore so will i need a reducer ???? if so can some one tell me the one i need . oh and what would be the best tooth count for a 1/10 scale running a 3900kv motor .any help appreciated guys thanks in advance ..