OK...I've got to replace my starter pinion gear and ring gear.

Stock engine.

Should I go with the 84/10 or 66/9 ring/pinion gear setup?

Don't want to go back to the 102/10 stock setup because I've heard how weak they are and don't want to have to do this again anytime soon.

 

You have a 7 year old bike with over 100k and you think that qualifies as weak?
Hoping to get 120k out of something different?
I don't think there is a single part on a HD, or any other bike that someone will not tell you is weak.

 

if it were me i would go with the 84/10 if your planning on it being stock..which i guess youare..do you have some teeth on ring gear that are worn..?i have to agree that over 100k ..doesnt sound weak..nice site by the way 8541hog.....nice work..