The clearance on taper rollers is obviously a question of expansion to some extent, the wheel bearings on the Vincent have a running clearance not unlike the front wheel bearings on most all rear wheel drive cars. However, the pinion bearings (taper rollers) in the "differential" of the same car do have pre-load, and this is so the pinion maintains exact alignment over the normally expected life of the vehicle. It is also exposed to a more fluid kind of lubrication (gear oil rather than grease) The very clever "Back to Back" taper roller bearing set up used in the drive side of the majority of Harley Davidson engines is near bullet proof, and will handle huge amounts of power and abuse.......Actually far superior to the stock bearing arrangement used in post war Vincent's and a mod that a lot of "Hotrod" Vincent builders use. These bearings come with a "Special spacer" that is designed to space the two roller races apart with a known running clearance which must be used. When assembled you can feel the movement in the crank, but only just. Certain brands of bearing are preferable to the known builders who have learnt which ones survive and which don't.
