If you lower the gearing of of the road going BSA by reducing the primary chain engine sprocket (18 teeth),the duplex chain side plates will not clear the inner crankcase seal housing. In my case I didn't realise this until trying to fit the chain!
I did consider making a new spacer to allow the sprocket to sit a little further away but this would mean doing the same to the NEB clutch and there is not really sufficient shaft length to do so.
A word with Terry at Avery Products told me that the accepted method is to fit a thinner oil seal and dress the housing away.
Ideally the housing should have been machined before assembly, however, by fitting an old oil seal and filling the area with silicone I was confident that using hand scrapers and rifler file I could remove and fettle the section.
I then checked the new oil seal, lip position by fitting the spacer with engineers Micrometer Blue around the outside. The seal lip only begins to contact the spacer, this will require a new spacer with a very slight counter bore to allow the unit to sit deeper and provide full lip contact.
It seems there may be a similar problem with the clearance of the drive chain against the gearbox casing. The gearbox sprocket has 13 teeth.