Hi all,
Thank you for this post and recommending Nils from WCW.
After seeing the other posts, I dropped off my son's cranks to be shortened to 150mm from 170mm with Nils and today he posted the below pictures.
We decided to retain the original 170mm holes for whatever reason (I think if he gets to 170mm I would just buy different cranks).
Definitely worth it for the younger one's.