OK, thanks for the explanation. I don't have a groove around the top of my 1987 15 HP driveshaft. I just wanted to make sure I wasn't missing something on the assembly.Depends on the year of the motor. On early 9.9/15 hp motors that went on the top of the drive shaft. Later models had 2 seals incorporated in the lower crankcase seal, So the o ring was no longer required. If it is required you will have a groove at the top of the shaft.