Reproduction cam chain tensioner which was used on some years of the Honda XL/XR250R singles. 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: 14500-471-000
XL250R - 1982-1983
XR250R - 1981-1982