Speedo drive woes, and small balls

Stewart Warner speedo drives fitted on 741 Scouts and 1948/ Chiefs my one was new and presumably pre-packed at $300+ I fitted it and after 4,000 miles it decided on the last Association ride to not work. It is a pain to dismantle  as you have to remove the wheel to un-bolt the small fibre drive gear then I find the fitted Zerk grease nipple has no little ball in it to stop the ingress of water or dirt, check your Zerk. The gears unlike the earlier Corbin rear drive versions are not easy to find and are $200USD a pair plus post so I ordered a new drive, expensive lesson learned.