This is the 30 mm Crankcase Cap for the CBR 125 & CBR 150. New OEM Honda Part Number 90087-KPP-860.
These are very useful to have on hand when lashing the valves as they are very soft and easily damaged. When installing them - do not over-tighten! They do not come with o-rings so be sure to transfer over your old one or replace it.
