I disconnected the flexplate from the torque converter and used bright light to check as best I could (only a narrow space between the plate and converter). Couldn’t see any sign of hairline cracks around the crank mounting bolts. I would expect to see a hair line leading outwards from the bolts if there was a problem.
This is the mounting hole for the TH350 transmission that came with the car, but they can’t be used for the TH250c
There are three mounting holes on the plate that can be used for a TH250C. My research indicated that this flex plate can be used with both types of transmissions. I bolted the converter to different holes and also pushed it up before tightening the bolts as the converter sags down when it is released from the flexplate. Not sure if that will help, but I need to correct every possibility for vibration.
Using my engine stethoscope I seemed to hear a noise in the water pump and I noticed that the alternator bracket was vibrating.
So I swapped the alternator for one I had rebuilt and was on the shelf – but no joy here.
So next was the water pump. The pump bearing was a bit rough and had s bit of sideways movement.
New water pump, but again no joy. Still the old pump was on the way out so nothing lost there.
Next: even more testing!