It's aircraft "Speed Tape" (the answer to many aircraft problems 😁) I use to keep the dirt out of the bearing housing but a recessed and sealed bung will work just as well. I found the swing arm needs to run as close as it can to the drive sprocket to get the belt to track without putting pressure on the front sprocket guide plate. If it is running hard up against the guide plate it will eventually fail, usually the screws break. A long straight edge between the front and rear sprockets, with the LH shock absorber removed, will help get the best alignment. You may have to sacrifice a bit of rear wheel alignment with the chassis to get the best compromise. I think Phil Bleazey recommends about a 1/4 turn loading on the swing arm bearings, it seems to work for me, no bangs or clunks. I have Phil's BB mount as well. There is always the chain drive conversion?
That's pretty much what I'm aiming at; minimal clearance between the swing arm and the sprocket. I've probably got slightly more than a 1/4 turn of load on the bearings, but it is quite difficult to judge just where that 1/4 turn should start! I've found that I can get the straight edge on from underneath without removing the shock absorber but it does involve getting the car jacked and supported up high then much rolling around underneath it with lights and the straight edge plus a healthy amount of head scratching to get things moving in the right direction.
I have some aluminium tape that is similar to speed tape that I may well seal up the swing arm with, it's a lot easier than making a custom recessed seal for it.
