I rarely drive my MK1 in recent years, and this is one reason why: I've probably replaced the alt/fan belt four times in the last five years. Original engine was replaced with a 1275A+ decades ago, but the belt-problem is relatively new. Belt I've got on it now, I think the same as ever, is a Dayco 15320, nominal 32" (32.57" outside circumference.)
My process is probably about the same as everyone's: installing a new belt, I struggle a bit to get it over the alt pulley even when fully slackened (I usually push the car backwards in 4th gear to bump it over), then tighten by swiveling the alt up until it feels snug but not taught, retighten the alt bolts (which haven't slipped). From that point, it charges fine and I hear no squealing, and I think everything's back to normal, and I'm wishfully proclaiming the problem cured.
A few drives later: voltage-gauge doesn't swing up out of the red zone, if it's in the evening, I notice headlight color is orangey, and maybe there's a tell-tale squeal. I'll check under the hood and find that, sure enough, the belt is rattly-loose, and there's a fine black powder accumulated around the water-outlet, from the belt being reduced to dust. I can retighten the belt once or twice, and all of the above repeats.
There's no obvious misalignment of pulleys, though at times while fighting this, I've been convinced enough that this was an issue to work in washers where it seemed they might be needed to improve the alignment. There's no room to put any kind of straightedge down into the works to verify what my eye tells me on this, but I think my eye is pretty good - and my gut tells me that even a significant misalignment would take a few thousand miles to show up as a badly-worn belt. I put maybe a thousand miles a YEAR on this car.
The pulley surfaces have, at times, appeared a bit rusty, so I went to the trouble to remove the belt and go over them with an air-driven fiber-disc, followed by epoxy coating. No difference.
Water-pump spins freely, as does the alt - which I changed twice during the problem-period.
I do notice one thing that I've not yet run down: rusty stains on the water-outlet bolts, as if it leaks under certain circumstances (never does so while idling with hood up, however). I'm wondering if under some driving conditions, it could be 'lubricating' my belt by squirting out some coolant/water, but I don't see any big drops in coolant reservoir.
Anyone been down this road before?
Mahalo - Dave in Honolulu