Not an MB, but distributor shaft gear pin keeps shearing.
 

My friend has an '84 Ford Econoline van. It runs good, but he has had the roll pin that holds the gear onto the bottom of the distributor shaft shear for the third time today. This last pin has only lasted about 400 miles. The second pin lasted for 8 months. He is just driving along and the van loses power. It mat backfire a little, but that is it.

What could cause the pin to keep shearing?

I'll assume it's a Ford V8 (302 or 351?) and you're using the correct replacement pin. Most likely cause is a binding / stiff oil pump. The distibutor shaft has a hex socket that drives a 3/8" (approx) hex shaft that goes down to drive the oil pump gears.
Of course something in the distributor shaft or centrifugal advance could be jamming but that's much less likely and you probably already checked that, right?

Happy Motoring,
Mark

It is a V8, but I'm not sure which one. The pins came out of other distributor shaft/gears. I'll let him know about the oil pump. Thanks.