I have a 4 inch cast iron drain line coming from the toilet which makes an immediate 90 bend and runs about 3 feet to the vertical stack. A tub is also connected to a 1 1/2 inch copper drain line that enters the cast iron toilet fitting just after the 90 degree bend in the 4 inch cast iron pipe. There has been a chronic clog at this junction. After removing the toilet and cutting off the 1 1/2 tub drain I looked into the connection and found a reduced orifice so that the 1 1/2 inch pipe only has a small oval opening at the top for the tub water to drain into the cast iron. I have not encounted a reduced openig diameter like this before. It this normal? Does it serve some purpose? The tub chronically drains slowly although the line to this point is clear and there is good pitch on the drain line. The 1 1/2 tub drain runs about 5 feet does an 90 degree turn and then runs about another 5 feet into the opening shown in the picture.