I've always used rubber sealed bearings and removed the inner seal...I do that on the advice of a friend who is also chartered engineer who worked for RHP bearings..However, other ideas may work as well....The primary purpose of the sealed bearing is to reduce the amount of oil that will leak through the bearing into the primary cases if the engine wet sumps after being left standing...BSA single engines don't suffer from leakage into the primary cases during normal use and do not pass oil there by 'breathing' into the cases...
Completely unshielded or partially shielded bearings do not suffer from any longevity issues and don't 'run dry'..Having never run shielded bearings I can't really comment on those...I guess 50,000 miles might answer the question of longevity on those....I've never had a drive side bearing failure and once owned an M21 that had done just over 100,000 miles from new without a bottom end strip down...Similarly, after a rebuild, I completed around 80,000 miles on my '51 rigid B33 before stripping the engine again and that wasn't for a bearing change...In total I have done hundreds of thousands of miles in the last 50 years on BSA heavyweight singles with both standard and tuned engines and can assure you, bearing longevity problems really are not an issue...Ian
