I notice that the retainer ring on my prop shaft seal had came off. I checked the oil and it looks fine. My question is, does the lower unit have to come apart to replace it or can the seal just be replaced. Thanks

im little confused on what part you are talking about if its #5 in the picture in the line below then you can replace the two outer seals behind the prop, they are a pain in the ass make sure the prop shaft is clean, if possible leave the inner one alone and just replace the outer seal, some of the spacers and clips are discounted.

http://www.mercruiserparts.com/Show_...9+%28IV+SSM%29

when you say retainer ring are you talking about the spring around the seal? on my #5s the seal is exposed behind the prop, there is no retainer ring.

My mistake, the spring on the outer oil seal came off when I removed the prop.