About 35 years ago, in my 390 powered 64 Falcon, I noticed a knocking noise coming from the engine. To me, it sounded like a rod bearing knocking,so I dropped the oil pan, and checked all the rod bearings, they all looked fine. So I pulled the engine to check more closely. When I went to rotate the crank with the flexplate, I noticed I could rock the flexplate back and forth a bit, without the crank moving. I was using a factory 390 flexplate with the reinforcement ring. When I removed the 6 flexplate bolt, and removed the reinforcement ring, there was a 360 degree crack, all around, and just inside the rings OD. A new flexplate, and she was good to go for another couple of years.