When I was pulling things apart for my rebuild I found a steel disk about 3 inches in diameter and an inch thick. It was just sitting on top of one of the frame rails and not attached to anything. At first I thought it was a part from some other machine that just happened to land on a frame rail and get stuck there, but I found a matching bolt and bolt hole in the frame. The thing is I cannot understand what this part is or what it does! I have rebuilt the cradle, rear suspension, and now engine and have never found a purpose for this part. Someone clear up this mystery!

Vibration damper

Yeah, it is a dampner for the cradle on 4 cylinder models. It hangs out on the cradle close to the crank pulley I want to say, 84 only?

It's on my '86 as well.

My '86 Duke had one of those, but the '87 didn't. Instead, the '87 Duke had a liquid-filled motor mount.

Okay that would make sense, it is an '84. I built solid cradle bushings a year ago, I wonder if this thing is still necessary. I'll have to do some testing when the rebuild is complete.

i was wondering the same thing about its real purpose...
tis not logical to me..

It helped a little with the vibrations felt in the body through the cradle. Seen these in other 2.5 GMs. A lot of A bodies. Was more of an Anti-complain thing.
When the cars were new, it helped ever so slightly. After they got a little age to them, they became paper weights.
Should have seen the size of motor mounts on early Quad4s.