Tensioner could be misaligned, you mentioned replacing both tensioners. And using your socket wrench you can move the AC tensioner with moderate force correct? If for some reason the tensioner isn't moving, it could be making the belt too tight which will overheat it and wear it very fast.

Only other thing would be the AC compressor is not lined up correctly. Has that been replaced, or any other work that would have for some reason required the compressor get R&I'd?

If the crank pulley were out of alignment the other belt would have broken also and your engine could possibly be rough running...

Look at all three pulleys on the AC belt carefully and look for any nicks/cracks/debris stuck in the grooves.

OEM parts meaning from the dealer? Should be Continentals if bought 3rd party.

P.S.
You should use the AC once a month for a whole drive cycle to keep all the seals from drying out and to keep the compressor lubricated. Plus it helps defog your windows quicker in the winter.

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Olympic Peninsula, WA, USA
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2005 Mini Cooper 100k, British Racing Green/Silver-Black
04/2002 4.6is 140k, Silver/Black-Alcantara *FOR SALE*
1989 325iX 270k, Diamond Black/Black
1984 318i 370k *SOLD* (First Bimmer)
Honorable Mentions: '71 2002, '87 325, '90 325ic, '92 525it, '93 325i, '94 530i