Hows belt riding on that tensioner pulley? Is it off the front even a little??? If it is, big problem. The belt must ride COMPLETELY on the tensioner pulley, and the tensioner pulley is only as wide as the belt with very little extra width. If the belt is not entirely riding on that pulley, it will shred in quick order. Ask me how I know this...

It seems there was a couple of different tensioners installed on that engine, and 2 of them got little shims between the tensioner and block. This is very easy to miss, and even 1/16" will cause the belt to shred.

Take a closer look at that and let us know. I dont see front edge of tensioner pulley in your pics, which is reason i ask.

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Paul
Benz-less
I need an SDL !