NSRs are all grey imports and were restricted from 89 onwards. MC18 and up as they came from japan. Story goes too many accidents were happening on the early unrestricted model the MC16(pre 89). Top ends are as good as up 40000km if synthetic has been used its whole life and the bottom is good for 60000km before the bearings even need a look. To be safe you'd prob do the rings every 15-20k to keep it all fresh. For more peace of mind they are based straight from the NSR250 GP bikes and are strong as hell.