Vibration problem solved
 

Car is a 65 GTO with stock 389. No A/C. Went to install a crank pulley and water pump pulley designed for A/C cars to improve engine cooling at idle. It did but that's another story.

I've always had a vibration between 48-52 MPH. I checked the normal things like U joints pinion angle and the other things you need to check. Everything checked good.

I removed the crank pulley and balancer from the balancer hub. I was shocked at what I saw on the crank pulley. The 6 bolt holes on the pulley were badly elongated. I also noticed that the 6 bolts that hold the balancer and the pulley to the hub were not very tight. The balancer holes and reinforcement plate holes were good. Put the new pulleys on and torqued the 6 bolts to 25 ft.lb as per the 65 service manual.

Went for a test drive and ,to my surprise, no more vibration between 48-52 MPH or any other speed for that mater.

I guess it just goes to show that with a 50+ year old unrestored car you have to think outside the boxAttachment 553826

nice catch
b4 the implosion occurred
the balancers also like to crack at the keyway too , seen on the seal side
zoooom zoom

Should be only 4 holes that thing was molested or from another make engine buggered up to somewhat fit

thats the style thru 67 on the early 2 piece balancers
68 was first 4 bolt and new balancer,,,

I dont have experience with early years just 66 and later, but it was sure buggered up

Nice find, it's good that you stuck with it !