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?
