The extreme valve / guide problems seem to appear mostly on newly built engines...this may be down to component quality but it may also be that higher mileage engines have polished and work-hardened and quite possibly built up a protective later following use with leaded fuel - although the 16H was originally developed in an era when that wasn't the case.
The guides were supposed to be lubricated weekly or every 500 miles which doesn't seem often enough to keep up a decent supply of lubricant to me.
An engineer mate of mine told me a while back that all lead replacement stuff was utterly useless. Anyway these engines are pre lead, as is my M21 and my Series 1
However as the previous version of this engine didn't have valve nipples one can only suppose that Norton added them to address a problem.
