Oh how I dislike hearing one of these dependable old engine's having growing pains. In the past I have had harmonic balancer problems. Think of a toy top trying to spin with a weight on one side, now think about a large weight on the end of the crank shaft. This can cause all kind's of vibration sounds through the MAIN part of engine. Everyone is telling you problems that has happened to them or are aware. If Hotrod Lincoln is correct (he has never been wrong for me), you could have a serious problem with a main bearing. I would go with the simplest first then proceed to the next. Yup a throw out bearing or a piece of metal inside your flywheel pan can be nasty. Ask Hotrod Lincoln about the possible crank bearing issue. It is always best in possible serious internal situations to drop the oil pan. If metallic fleck's are present you have a solvable problem. Good luck. Let us know, that is how we learn. Doc.
Last edited by Doc.Hall; 07/06/2020 12:50 PM. Reason: misspelling
