Thanks blue68cat and the others that suggested the torque converter. Once I removed the TC and bolted the bell back up, it ran smooth, so the TC is out of balance. This is a rebuilt C4 and new TC but there are more problems with the trans than just the TC. Before the TC was removed, I made sure the trans was full of fluid (I filled the TC before it was installed) and then ran it through the gears. There was major grinding going from R to P, slight grinding from 2 to 1, pan leaks and loose tailshaft bolts. Looks like it is going back to the rebuilder. Thanks again for all your advice and help.
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I had an imbalanced TC on my 73 after the trans rebuild. In my case the trans rebuild was the only major change, so that quickly narrowed the problem down. Unfortunately the carb was gunked up as well, so after confirming the vibration was not engine induced (the ignition parade pattern showed the spark timing was identical from cylinder to cylinder), the TC was the next candidate.
Fortunately, my trans rebuilder was very good about fixing the problem without hassle. They had the car for less than a week.
This leads me to a suspicion that like many replacement parts, replacement TCs are often crap.
Does anyone rebuild, TCs?
Just add a few things. Junk parts are just a fact of life these days. Flexplates, dampers and converters not balanced or balanced wrong are not uncommon. Any V8 getting rebuilt should get balanced. Any time you buy a flexplate or damper have it spun up and checked. Not uncommon for them to be off. Torque converter buy one from a good company like Precision of New Hampton.
As for C-4’s there are two different cases. The step case where the dipstick goes into the case. It uses the 157T flexplate and the bell is attached using the pump bolts. Then there is the flare case which has the dipstick tube go into the pan and that one uses the 164T flexplate and the bell is attached with its own set of bolts separate from the pump.
The step case was used with the 289/302 in cars. The flare case was used with the 351w/c/m from the factory. You will also find the flare case in trucks behind the smaller engines. There is also a difference for all C-4’s regarding the input shaft and some internals with 64-69 being 24 spline and 70 and up being 26 spline. They require different converters.
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I had this it was u joints at the back
exact same symptoms