I'm happy to report that on the 1994 E420 the harmonic balancer can be walked off the hub with the 27mm bolt + 4 washers on, no need to remove in order to change the water pump. You do need to hold the 27mm bolt in place while loosening the 6 13mm bolts holding the pulley and harmonic balancer in place, but only to prevent the crankshaft from turning counter-clockwise. Also, the pulley and harmonic balancer only bolts onto the hub one way, because one of the 6 bolts is a little off center.

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2004 E500 4Matic 138,000 miles