I'll make the story short:
-Installed Mr. Amp alternator
-put new belt on
-drove to store and got gas...belt broke before I got home
-alternator pulley was out of line from water pump and crankshaft
-ground spacerdown until all pulleys lined up with a straight edge
-new belt different day...belt is still slipping whenever engine is revved up
-I was told by the makers that there should be no more play than 1/4 inch in the longest part of the belt.
-the pulley has a deeper groove that is supposed to help the belt catch harder

Here are my questions - When checking play in the belt, is it measured by the total up-and-down motion or just the distance by pushing down on the belt?

The people who built the alternator said to have 1/4in. play but the recommendations on my aluminum water-pump suggest 1/2 inch...which oneshould I follow?

I have a Dayco Top Cog Belt, would there be a better belt for my needs?

Are there any other reasons why the belt would be slipping?

 

I've never had much luck measuring the play, if it's slipping and everything is lined up it's just too loose. See if you can turn the alternator pulley against the belt by hand. Tighten it until you can't. Then give it a little more and you should be fine.

 

I was taught the twist method. Should twist no more than a 1/4 turn

 

Be certain that the belt you are running is not too thin...

The belt should be wide enough that it rides on the "V" of the pulleys, the belt should never run on the bottom of the groove.

If the belt is riding right, it should not slip easily. If you have to pull the belt really tight to make it not slip, you will run the risk of over stressing the bearingsin your belt driven accessories.

Catch phrase here might be "a chain is only as strong as it's weakest link". In other words, a super tight belt might shorten the life of your water pump.

 

I put a Dayco top cog belt on a while back too, when I still had my regular alt. It would squeal a little a start up but it was tight.
I changed to a 1 wire Alt & it squealsa little worse now, but it is tight.
Since we both have this problem & we both have Dayco top cog belts, I say the belts could possibly be the problem.
I'll get another brand (a regular looking V-Belt this time) & try it out. My car is not driving right now, but I can start it & rev the motor some & see (or hear) what happens.
I'll post the results.