How long do they last? Is their longetivity a function of time? Or is it mileage? Thanks in advance.
Thom

Well, it's dependent on both time and mileage. Also temperature will play a part in how long they last. I've replaced them at under 50,000 miles(warranty work) on some cars. Why do you ask? What model do you have?
Regards,
Randy D.

Randy,
Thanks for the response. I've just taken possesion of a 1969 280 SEL and the thought just crossed my mind. Actually it was my experience with a 1972 220 that made me ask. It leaked oil like a sieve and left me permanently traumatized. :D
Thom

Hi Thom,
Okay, I understand now. On the older cars(like mine)the rear crankshaft seals are considered a last resort unless the engine needs a lot of work anyway. I think anything over 150K to 200K miles is lucky. But they can "seep" for years and years. They usually have to be leaking real bad to warrant replacement.
I pulled the engine out of my 6.3 two years ago to fix the rear seal with 140,000 miles on it. Mine was leaving puddles. Of course I needed to get all the stuff on the engine re-plated and replace some other pieces as well, but I really don't like the old "rope seals". They are a serious pain to do!
Regards,
Randy D.