The only thing I can figure is, it's cheaper to drill a hole and punch a ball bearing in it then it is to drill and tap (two steps) for a plug. Witch also requires machine work. If the same method were used as for the front, it would also require facing for the copper washer to seat to. I thought the same thing and this was the only explaition I could come up with.

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Jim
82 300SD saved from scrap yard, 86 front clip,85 500SEL rear end,86 wheels.