I have had SO much trouble keeping the crankshaft / water pump belt adjustment correct on my 82 diesel westy.  I get the belt tight by removing shims on the water pump pulley and everything works ok for a few hundred miles, then it starts slipping.  I've replaced 3 belts (all high quality Bosch) and one even wore so badly it lost all of it's "wedge" becoming a rubber band.  

I first smiled at the ingenious way the VW engineers figured out how to adjust the water pump fixed solidly to the block with a single belt.  But removing the correct number of shims is a hit or miss guessing game and is difficult to do in a tight space.  I try to get it as tight as I can, with maybe 1/8" available deflection at the midpoint of the belt span.

As far as I can tell the water pump shaft bearings are still good (no leaks or noise)

Has anyone had similar difficulty and figured out a solution?  Or am I just not "getting" something and making an obvious mistake somewhere in my "water pump belt adjustment" logic?

Henry Jackson
82 1.6 diesel Westy
Palo Alto, CA.