The oil sight glass allows the user to check the oil level in a pump. If cloudy, clean or replace. The oil level should be right at the red dot in the center of the sight glass, or right in the center of the red circle (depending on the sight glass). If above, drain some oil; if below it, add oil.
This item fits the following models:
15-pc Thunderbolt Black Oxide Drill Bit Set
1/4" x 50 Ft Noodle Air Hose with Coupler and Plug