About 5 years ago (6,500 miles), I noticed a vibration in my 1969 280SL, always at about 35MPH. I did an inspection, and discovered that the flex disc was no longer serviceable. All of the mounting bolts/nuts were there, but some had become very loose, to the point that they had damaged the holes in the drive shaft mounting flange.
I disassembled the unit, and had the driveshaft refitted with a new mounting flange and rebalanced. The output drive shaft of the automatic transmission was not noticeably damaged, and the grooved nut was tight. I did not test for torque on the grooved nut (a special socket is required, which I do not have).
I bought a new flex disc kit (with nuts/bolts), center cross, and installed the new rebuilt/rebalanced drive shaft. Before assembly, I read all that I could on installing the flex disc, and copied photos of the correct installation.
Initially, she ran smooth, but about a month ago I noticed a very small vibration, so yesterday I placed her up on the ramps and inspected the drive line. I discovered that one of the short flex disc mounting bolts had worked out completely, and was laying on the transmission mounting plate. The nut was gone, but the washer was found next to the bolt. All of the other nuts were tight. I replaced the missing nut with one that I had saved from the previous flex disc replacement (exact match), and bolted everything back up. I also checked the engine mounts (passenger side was loose), and the transmission mount. She rode smooth again.
The engine and transmission mounts are new. All rubber at body mounting points are new (sub frame, trailing arms, springs, etc).
Obviously, something is/was not correct, but I cannot figure out what it is. I don't want to do this job again; does anybody have any ideas?
