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Sheered pin on dristributor gear 2 times

Engine

Vehicle Information: Year: 1978; Body Type: Targa;

I have a 1978 Euro-spec 911SC with 480,000Klm on the car and 200,000Klm on a the rebuild of the original engine. The distributor drive gear on the crankshaft was in tact at the time of the rebuild. After a fairly trouble-free 10 years on the rebuild, the distributor gear sheered the distributor pin. It was replaced once, only to sheer a second time after about 50 miles of use. It failed under light load, about 10 minutes after some hard driving. When the 2nd failure occurred, I paid special attention to the rotor's position and condition while still in the car. The rotor was stuck, and would not budge by hand in a counter-clockwise; only when it was turned counter clockwise did it move. Then it turned freely 360-degrees in both directions. After removing the distributor, as suspected, the pin had sheered. My mechanic put a scope down the hole and said he saw some wear and/or damage on the crankshaft's distributor drive-gear. A temporary US-style distributor is now in the vehicle. The car runs, but there is a bit of a clatter coming from the general area of the distributor. I don't want to believe I've got to split to case to fix this issue. I'd be willing to live with the damaged crank drive until the next rebuild if I could get over the reliability concern of another sheered pin. I'd like to confirm the diagnosis. Is this a distributor that seizes intermittently? Could damage or wear on the drive gear cause the distributor pin to sheer? What other diagnostic steps should be taken to confirm the remedy (given the cost of drive-gear replacement)?

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