New BSA A50 / A65 unit twins primary chain adjuster cap. This cap threads onto your primary chain tensioner bolt at the bottom. Cap is threaded internally. Repalces BSA part number 68-0377 to fit all BSA A50 and A65 twins (Lightning, Royal Star, Thunderbolt, Hornet, etc) from 1962-1972. Check your parts book to ensure the best fitment.