Hi Larry,
Was the replacement bolt tightened to spec? The crank bolt should be torqued to 270-330 nm (199-243 lb-ft). If it's torqued that tight, it won't come loose. If the threads are damaged, it won't hold that much force.
Regards,
Randy D.

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W.R.Durrance
ASE Master Tech
Mercedes Tech 20 Years
68 250SE
68 SAAB 96 Ice Racer
69 300SEL 6.3
70 300SEL 3.5
81 300TDT