SE218 Lower Unit Mercury OB Counter Rotation | SEI 90-218CRSE218 Lower Unit Mercury OB Counter Rotation SEI 90 218CR SE218 Counter Rotation Replaces Most Mercury 3. 0L Counter Rotation Lower Units Do not use for standard rotation 13 Splines 25 and 30 inch Shafts. If you have an 8 Spline drive shaft you will need to buy the stub shaft unit and purchase an upper driveshaft from an OE dealer. 25" 45 823901A 1 30" 45 824115A 1 (This shaft is no longer available from the OE. You will need to either source a used