^^ If you're going this route, I'd recommend purchasing the entire GY6 pulley assembly anyway, as the NU50 only uses 2 guide pins (as opposed to 3). The bore of the ID should be close enough to bolt on, and the extra +1mm in diameter should have a de minimis effect on top speed. CW's post is focused on the variety of angles available for this setup (which, on top of those already stated, is a big perk to the GY6 over the DIY method).