These sight glass gaskets are designed to seal the oil casing to sight glass on Leybold D4B, D8B, D16B, D25B, D40B, D65B, D16BCS, D25BCS, D40BCS, D65BCS rotary vane vacuum pumps. When a vacuum pump is used in corrosive applications contamination can build up on the sight glass. This makes it hard to see the oil level. One these occasions the sight glass needs to be removed and cleaned. These sight glass gaskets have Leybold part number 23973057.