I apologize for the length of this post, but wanted to update everyone on the search for a solution to the vibration problem I am having. To refresh everyone, the car is a 1970 Challenger R/T, 383/727 that is a stock restoration with about 350 miles on it. After checking all the easy stuff that I could think of and that you suggested, the conclusion was that the source of the vibration was likely in the torque converter.

I finally found the time to replace the converter and used a new flex plate. The vibration did not go away. I did not run the engine very long and drove it only a few miles, but it seems like maybe it is slightly less, but still noticeable at around 1250 RPM in neutral, park, stationary in gear, and going down the road. I used a rebuilt stock 10 3/4" converter that I got through Pat Blais. The converter that was in the car was also a rebuilt stock 10 3/4" that Pat supplied. Pat has them rebuilt at a converter shop near him. There were no obvious issues with the converter that I removed, small weight was intact, inside of converter looked clean. There was also no sign of any issues in the trans when I removed the pan to drain the fluid. The trans mount was carefully inspected when I removed it to pull the trans and found no issues or defects.

Before replacing the converter, I removed the engine oil pan to inspect the bottom end. When the engine was balanced, the replacement pistons were lighter than stock, so material was removed from the crank. There was no mallory metal that could have come loose to unbalance the rotating assembly. Everything there looked as it should. Checked torque on the mains and some of the rods and everything was fine. One thing I noticed earlier was that the vacuum at idle was 15". I pulled the carb and replaced the thick spacer gasket that was on the manifold. The spacer is a FELPRO 60123. It has brass grommets holding two thinner pieces of material together to make the approximately 1/4" thickness. The flanges on those grommets sit slightly proud of the gasket material and I think were preventing the carb from completely sealing to the manifold. I removed the grommets used some #2 permatex to glue the two pieces together and put it back together. The carb is the original R4368 Holley that I refurbished when doing the engine build a few years ago. It was clean and in good shape. After restart, the idle vacuum was now 17", but the vibration remained the same. The engine runs strong up to 5600 RPM or so, idles smoothly at 750 RPM, starts easily, does not overheat, does not make any unusual noises, and I cannot see or feel any vibration in the engine like I can in the steering wheel and through the seat.

My thinking is if the vibration is originating in the engine, but being felt in the steering wheel and seat it has to be transmitting through the motor mounts. There are not any other parts of the engine in contact with the body or chassis of the car. I checked the mounts for tightness and inspected the rubber as best as I could earlier and felt they were fine. Might be that I need to remove them from the car and do a better inspection. Anyway, I'm quickly running out ideas so any thoughts or suggestions are very much appreciated.

Regards,
John