1979 Super Six Aspen with 904 tranny. I've read the posts about vibrations but cant shake(pun intended) mine. Happens around 60-65. I had another 79 Super Six Aspen before this one and it did the exact same thing but just a little worse. On the previous Aspen I changed all the 3 engine/tranny mounts, replaced the U-joints, that didnt work so I replaced the whole driveshaft then all the tires then I gave up. All this had no effect. Next car same thing. Well didnt bother with all the previous stuff on the new car went right for the leaf springs as suggested by Dan. Replaced them with NOS springs from John R Spring Service. Yes they were direct replacement NOS 4 leaf springs with Pentastars and Mopar part numbers too boot. DANG. No change in the vibes at all. Handles and rides alot better though. Definately worth the money and the 4 hours to change them. But dang it what gives with these cars. Is it the freaking lock up torque converters or do all slant sixs buzz the floor boards at highway speeds?

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When you replaced the whole drive shaft that means the slip yoke too right? Ever replace the tailshaft bushing in the transmission? Also, if the iso-clamp leaf spring bushings are shot and allowing the axle to rotate you'll get vibration.

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Joshua

The first car had 182,000 miles when I sold it and this one has 74,000 on it now. Had 71,000 when I got it. I thought about the tailshaft bushing on the first car because of the mileage and it was the orignal tranny. It was "soft" when I got it but came back to life after adjusting the bands and a new filter and fluid flush. But this tranny is stout and fluid looks good in it and I really doubt the bushing could be wore with the mileage it has on it. The iso pads looked "good" when I changed the springs Saturday. I cleaned all the loose scally rust off the shock plate and underneath spring box. Sprayed the iso pads with silicone spray and everything went back together great and squeezed together tight. I really thought I was going to get rid of it(vibs) this time but NOPE. The old main springs were "opening up" where they curve around the front mounting bushings and were curving up the wrong way right where the next spring underneath makes it frontal contact. It did tremendously improve the steering response and help the ride quality but no dice on the vibes. Whatever it is causing the vibes it has to be something these Aspens share in common. And it sure isnt something I've changed yet, obviously. All to eliminate this from your diagnose the first car I rebuilt the complete front end and this car was aligned and no wore out front end parts were present. They both went down the road straight as an arrow. And at 55mph its a smooth ride. More you speed up the worse it gets. Its not the nature of a slant six thats causing this floor board buzz is it? I had them years ago but when your younger you really dont pay much attention to stuff like that.

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Check and correct the pinion angle.

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

But how could the pinion be that far off? They are factory replacement springs. Its the stock rear-end. Is it the ISO pads? Dan when you had your trouble what caused your pinion angle to be off in the first place and how many degrees of shim did it take to fix it?

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Stackup of sloppy production tolerances, or just plain rotten luck. It doesn't take much rear U-joint angularity to create vibration. Mine was cured with...I can't remember, somewhere between 1° and 2°.

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

Whoa, really, that low? Well that will be easier to try then changing the torque converter. So the engine/tranny angle is what the pinion angle is supposed to be? I really dont care what the driveshaft angle is right? Put my angle finder on the harmonic balancer and then on the face of the yoke for the rear end and make them the same?

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Does this sound right to you Dan?

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Wow, that's a strange set of instructions. I suppose it might work, but the more I think about it, the more it might not. The main object is to minimise the angularity between the pinion nose and the driveshaft (rear U-joint) and between the trans output shaft and the driveshaft (front U-joint). The FSM procedure does have you measuring the driveshaft angle, not the crank pulley.

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

Yup. many of the Chryslers Ive had with lockup TCs have had that, as long as all else was OK Ive lived with it just drove a couple MPH faster or slower than the speed that it shook at, I tend to hang onto my cars longer than most, so I have done several trans swaps (for other reasons than this problem ), of course with a new (or reman) TC every time since you can't get all the old crap out of the TC regardless how much it gets "flushed" and its smooth after the swap. A reman TC for that car should be <$50 at a regular trans shop; cash/carry price, but there will probably be a core charge til they get the old one back. All in an afternoon's worth of work.

I had a 68 Valiant that buzzed after putting a 904 with a lockup converter in it. The buzzing would happen when the converter went into lock up. The 904 that I had in it before the lockup version had a shift plate and a couple of other tricks done to it and it shifted really hard. What it turned out to be was the holes for the u joint in the slip yoke had been opened up a little bit by the hard shifting 904 shifting hard. 1 new slip yoke and 2 new u joints and no more buzz. The u joint would slide back and forth in the old slip joint. Make sure you don't have any slop in the u joints and that includes the holes in the drive shaft and the slip yoke. Also make sure that the u joints aren't binding. New ones might be a little stiff at first but there should not be any binding.

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Former bent 6 owner
Mark

have you had your driveshaft balanced? mine was way out. made a big difference in the cruising of the car. didn't know it was as bad as it was until it was gone.

zedpapa

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1970 dodge dart w/225 /6 bored .040" over, holley 390cfm w/vac. sec., compcams 252s, clifford shorty headers w/2.5" exhaust w/flowmaster, f-body 11" front discs, aluminum A-833OD, 8 1/4 w/3.21 SG
soon to have 5 gears!!!