It's worth adding to this thread that a 52mmID x 2.5mm O-ring is a good replacement for the outside seal. The original ALDA on my SDL didn't hold pressure at all. Leaked at the 2 halves and out the shaft seal. The outside seal that seals the 2 halves together was rock hard and shattered when touched. New O-ring did the trick. If taking the ALDA apart to replace the shaft seal, it seems to be a wise idea to replace the large O-ring at the same time.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]