I can see how this is a pretty critical step. I spent an extra 10 or 20 bucks (don't remember exactly) when ordering my kit, and got the special tool for that lock ring/nut, so I would not have to mess around trying to tighten it with something not designed especially for that purpose.

The steel plate that goes between that ring and the bottom bracket shell and has two smaller holes to bolt it to the motor housing... that is one part that was not made to perfect dimensions in my kit. It would not quite fit in position without a little modification, and it's nothing to do with the bike. I took a Dremel tool to it and removed a little metal in the right spot, then it worked fine. Just an extra five minutes of work in an otherwise really well designed system.

The most time consuming part of the whole installation was placing a few little sensors (a Hall effect unit on the left chain stay, for bike speed sensing, and a little unit that the shift cable passes through, for shift sensing) and routing all the wiring neatly. You definitely want some zip ties for this kit.

There's a tiny magnet that attaches to a spoke to trip the speed sensor mentioned above each time it comes around. Works just like those cycle computers everyone had before GPS came along. The screw to hold that magnet on has a tiny tamper-proof Torx head, requiring a tool that I had neither in my collection nor the ones provided with the kit. I just cheated and grabbed the screw head with a vise-grip plier.

The only other snag (quite literally) was the screw coming through the bottom bracket shell to secure the plastic shift cable guide. This is part of the bike, not something specific to the Bafang kit. It protruded inward too far to allow the motor to slip through the shell. I had to remove it, shorten it a bit with my Dremel tool, and reinstall it. Then everything worked together just fine.

My only complaint with quality control, besides the aforementioned lock plate, is regarding the cheap plastic chain ring guard. It attaches to the chain ring with five tiny sheet metal screws that just tap into pilot holes in the plastic. One of them did not ever grab. I used gentle force and was careful not to overtighten, but I don't think that screw is doing anything. The guard is essentially held in place with only four screws.