I have it installed and looking at the the 39mm boss it was to long. It was covering some of the splines. So I used the Kymco 38mm smooth boss that came with my scooter. I also noticed that the rollers were the same weight so I may look into going with a 6 gram roller depending on how things go. Both the Polini and stock rollers were 6.9 grams
Am going to have my first ride this morning with it to work. Will let you know how it goes.