Be sure your belt pulleys are in good shape, if they are worn into some shape other than a clean "V", then the belt is likely to slip.
At one time I had a bad generator pulley on my truck, worked fine other then the belt slipping....

Grigg


•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-