Brace yourself for a dumb question ... Can anyone tell me what the difference
is between the 30mm and 40mm hub on the water pump for the 1.6L NA diesel?
(Please don't say 10mm!)

The Vanagon diesel requires a 40mm hub on the water pump.  I've always thought
the dimension referred to the diameter of the surface that the pulley attaches
to (the "hub", right?), but I have 3 water pumps on my work bench, one from my
'82 Westy diesel (and I know it is not original), one from my '82 Passenger
diesel (and I know the whole engine is not original) and one from an '84 Jetta
Diesel with no A/C (which supposedly also takes the 40mm hub, but I know it is
not original also.)  The "hubs" are all the same diameter, but the vanes are
different.  One has formed steel vanes (kind of like bent fork tines) and the
other two have a cast vanes (and they are not identical).  Am I just looking at
different manufacturing approaches to the tines, or have I misunderstood which
hub the measure refers to?

And finally, why does VW use two different hub sizes?  I think it has something
to do with the pulley drive system - as the Vanagon application uses one belt
to drive the water pump from the crankshaft pulley and a second belt to drive
the alternator from the water pump pulley.  On late 70's and early 80's diesels
with A/C, there is a single belt that drives the water pump and A/C compressor
from the crankshaft pulley and another belt that drives the alternator off of
the A/C pulley.  These all seem to spec the 30mm hub.  Is the difference
because of the way the pulley is attached?

Thanks for any insight.

Maynard Southard  (OvO) '79
'82 Westy Diesel "Reinhardt"
'82 Vanagon GL Diesel "Sluggo"
'93 EV GL "Klinger"
'00 Golf GLS TDI "WooHoo"
'84 Jetta GL TD "Donor"