On this forum people have occasionally reported the same. It is also not uncommon on other makes of alternators.

Personally I don't mind degreasing the Fan and Alternator Shaft with Brake Cleaner and putting a little slow curing JB Weld on the Shaft and I generally use an electric or pneumatic impact wrench to tighten rap the nut with no hold on the shaft or nut.

JB Weld is good for about 250-270 degrees F. That means when it comes time to remove the Far it is easy to get the JB Weld to release if you use a Propane Torch on the Fan. The JB Weld softens or burns and the fan comes off. A wire brush takes off the old burned epoxy.

People that had rebuilt alternators when the got the Car and later took them off to replace a bearing have said there was no Key and they did not suffer any pinning of the pulley. However, I have not seen anyone post a specific torque for the shaft nut and I have never looked for a torque.

I slso use loctite on the shaft nut. But, I normally am reusing the shaft nut.

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84 300D, 82 Volvo 244Gl Diesel