Are the pulleys lined up with each other?

Is the belt wide enough that it fits tightly on the sides of the pulley and not all the way down in the bottom of the groove?

 

yes it sits up a little higher actually about 1/16 over the top of the pulley plus i pretty sure its a standard belt, but they keep stretching out

They pulleys might not be perfectly aligned thats a good point. and the fan pulley has a slight bit of lop sidedness that you only see while running.

 

i was thinking maybe all new pulleys and switching to a wider serpentine style belt might solve the prob

 

Switching to a serpentine belt would certainly solve the problem, but it would be an expensive and probably unnecessary solution, I think. You seem to have found the probable source of your own problem - the lopsided fan pulley. A lopsided pulley would wobble back and forth and cause a lot of tension on the belt, and is probably what's repeatedly snapping it. Replace the pulley and make sure everything lines up right, and you should be good to go

Out of curiosity, what brand of belt are you using? The ones from Advance / Autozone are absolute crap, but the ones from NAPA have been great for me so far.

 

Out of curiosity, what brand of belt are you using? The ones from Advance / Autozone are absolute crap, but the ones from NAPA have been great for me so far.

Exacto-mundo! Kelly is spot-on. The stuff being peddled by Advance/Auto-Zone/O'Reilly's is based on price point. A lot of naive drivers want whatever is cheapest. So that's what they sell; not a good idea for your Mustang.

Dollars to doughnuts a set of NAPA premium belts will eliminate your problem. But you should also replace any warped are otherwise damaged pulleys.

 

yeah the autozone or o'rielly ones - ill try the napa here in town tomorrow

 

My first thought was alignment of the pulleys. Take all the pulley's off and lay them down on a smooth flat surface. See if there is any wobble or warp in the pulley's. Also check for proper mounting. Did all the bolts holding down the pully to the shaft get tightened down the same distance. Are there extra spacing washers between the pulley and the pulley shaft? Is the bracket holding the alternator or a/c or power steering mounted correctly so that the belt can align correctly? Are you running a clutch on the fan blade?
That belt is under some kind of stress, either from the pulley's being mis aligned or too much or too little tension on the belt. You say that this has happened several times. How often does it happen (months/weeks/miles driven)? Have you had the car a long time and this just started? Has some other type of engine work been done and the problems have happened since then? Even the worse belts you get shouldn't break repeatedly. Just changing manufacturers or distributors is not going to keep a belt from breaking repeatedly, you've got to find the root cause and I don't believe we have enough information yet to fully diagnose your problem.

 

Another thing to check...Could one of the pulleys have a nick or gouge that's sharp and cutting the belt?

 

+1 on this , sharp burr will eat belts fast

 

If you have a pulley that looks "lopsided" it's either misaligned or the accessory has a bad bearing and/or the belt is being overtightened. Most belts will have 1/2 to 1 inch of deflection halfway between the pulleys. Overtightening is the most common reason for water pump failure that I can think of.

 

I have had problems with napa brand belts they seem to need readjustment after driving ahile, Gates or Goodyear are better brands. Can you post some pictures of the broken belts?

 

I'm surprised to hear this. I've been helping a friend put a new engine into his '68 GTO. (I know, "Blasphemy!") He used to dog that 400 Pontiac hard. (So hard, he blew it up. Hence the new engine.) Anyway, while bolting on the accessories I noticed all his belts are NAPA and he never had any issues with them.

 

My p/s pump pully used to be off by a couple of degrees which caused me to spit belts on downshifts.

Check your alignment using a long straight edge.