Reproduction cam chain tensioner which was used on many of the Honda XL/XR500 singles from 79-81. As your engine runs the cam chain is spinning all the time and it slowly wears all of the parts that guide it and put tension on it. If you're doing your top end and you plan to replace the cam chain and it's associated parts, the tensioner should be replaced. Check for wear of course, but sometimes it's good to go by the "while you're in there" approach.
Replaces Honda OEM Part Number: 14500-429-000
XL500S - 1979-1981
XR500 - 1979-1980
XR500R - 1981