There have been quite a few folks on here that had problems with AAZ crankshaft keyways. It's a world-wide phenomenon.
As noted by others, the combination of a harmonic balancer, and a fixed-pulley on the alternator puts a good deal of stress on the crank sprocket.
If the crank bolt is not properly torqued when removed/installed (and even sometimes, when it's never been touched), the sprocket can rock back and forth in it's keyway. Through time, this will wear the sides of the crank keyway, and in the worst case scenario, jump time and do damage to the valves and pistons.
The ideal and permanent fix if it ever happens to you, is to do any one of several tricks:
1) Remove the crankshaft, and weld the nose (sprocket end) of the crankshaft. Then, have it broached to accept the TDI sprocket, which has a flat spot and no keyway:
http://www.msnusers.com/gtdforum/shoebox.msnw?Page=32) Fix it on the car using a jig and a broach, as outlined in the link above.
3) Use metal dowel pins and drill into your crank nose and sprocket to secure the sprocket and prevent it from rocking. There are several write ups on Vortex about people who've used dowel pins on crank noses (usually 16v or G60 cars).
http://forums.vwvortex.com/zerothread?id=3793916If you want to read up on some cases, check out the following links
http://forums.vwvortex.com/zerothread?id=3959069http://forums.vwvortex.com/zerothread?id=3988582http://vwdiesel.net/phpBB/viewtopic.php?t=1010&start=0 http://vwdiesel.net/phpBB/viewtopic.php?t=2719 http://vwdiesel.net/phpBB/viewtopic.php?t=5874 http://www.vwdiesel.net/phpBB/viewtopic.php?p=14722 http://vwdiesel.net/phpBB/viewtopic.php?t=17422&highlight=The last one is a recent AAZ crank nose failure on a UK Vanagon.
What this has to do with your ABF setup is that, like the AAZ, you'll be running a heavy harmonic balancer, and an alternator, that at high speeds, will have a lot of rotational inertia. So, when you let off the gas, the motor slows down quickly, but the alternator may still be trying to rotate forward after the engine has already slowed down. Because of the added-contant tension on the belt from the spring-loaded tensioner, there is no way for the alternator to release that inertia energy.
The VW factory solution was to install a pulley on the alternator that only spun one-way (as posted by Vince above). When the engine is not pushing the alternator, it free-wheels so to speak, when the engine powers down. This relieves the counter-acting forces which put stress on the crank-shaft pulley.
I know it might suck thinking about putting another $150 into an alternator pulley, but if you look at any of those threads listed above, you can see that the alternative is much worse.
Just a thought.