On my Nutrail the drive gear is offset and there is a gap exposing the driveshaft and casing to crud and water. ( I haven't looked at other 'normal' sprocket set ups).
I purchased a soft rubber pipe grommet from a local plumbers merchant that I then trimmed down to fill the gap and overlap the plastic casing as shown below;
I purchased a soft rubber pipe grommet from a local plumbers merchant that I then trimmed down to fill the gap and overlap the plastic casing as shown below;