What is the purpose of the big weight on the tailshaft of some 904 transmissions?

Just helps dampen vibrations from the driveshaft,just a mickey-mouse way for chrysler to slop a poorly balanced driveshaft on the assembly line without the customer complaining.

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V8?? We don't need no stink'n V8!!

It's an anti vibration/anti-bounce weight, I had one on the 1973 Charger I took to the demo derby... it was kind of weird to see it , considering most other A/B/C/F/M/J body cars I've seen didn't have them...

If it's an A-body...ditch the weight...I had a drive shaft recently balanced...it cost a whole whopping $60... it was worth it.

-D.Idiot

Nope, wrong. That weight would not compensate for an out-of-balance driveshaft. It's a harmonic damper to reduce a thrumming noise that has nothing to do with driveshaft balance.

If anyone happens to have a spare such weight, with good rubber bushings, I'm after one to add to my '73.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Interesting. I've only ever seen a few. I'll hang on to this one, and keep an eye out for another one for you Dan.