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Dear Greg,
Good p.m. and may this find you well. An alternator belt should last more than 70K, unless it is (a) 11 years old; (b) has been too loose or (c) has been very much too tight.
If the belt is the original, it may simply have dry-rotted from age. Do you have maintenance records, which show when the belt was last changed
Being too loose allows the belt to slip, which generates heat, which softens the belt and leads to failure. Being too tight over-stresses the belt. Generally, being a bit too tight is better than being a bit too loose.
Check the alternator bushings (these are the three round rubber grommets through which the alternator mounting bolts pass; two at the top, one at the bottom of the mounting). The bushings should be round. If the bushings are egg-shaped, the belt has been over-tight.
The bushings provide vibration insulation and keep the alternator's axis exactly parallel with that of the crankshaft. That means the drive belt is perfectly perpendicular to the axis of the generator's shaft and to the axis of the crankshaft. This is how it should be. When the bushings are distorted, the alternator shifts, and that puts strain on the belt, one side of which is pressed against the inner surface of the pulley. That accelerates wear.
Replacement bushings can be bought from www.fcpgroton.com. With only 70K, get the Urethane bushings. They're more costly than the rubber originals, but will last a very long time.
When you re-install the belt, inspect the surfaces of the alternator pulley and the crank pulley (the harmonic balancer) to see if there's any black rubber residue. If there is, that's evidence of belt over-heating. Remove the rubber residue with paint thinner or methy ethyl ketone (MEK, chemical cousin of acetone). Wear gloves while handling these solvents. Dry thoroughly before installing the new belt.
Hope this helps.
Yours faithfully,
spook
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