I'm wondering if the mts had a slightly different thickness & changed the ujoint angles a bit or if they are just stiffer. the noise got worse with the gear change so that would point me there but what confuses things a bit is the shaft balancing helped. I would check the angles & confirm that the front yoke moves smoothly by hand back & forth in both planes & the rear ujoint is smooth in its one plane. too many changes at the same time does complicate diagnosing. Keep us updated on any info (we'll get it)

& another potential (I had that once) is not enough front yoke engagement into the trans (& or bad ext housing bushing). Mine was a PT vibration at random sort of (mostly deccel) & I swapped the correct shaft in & all good.

If the tailshaft bushing in the trans has wear, should I be concerned until I can get the trans out and freshen it?

I ain't for sure & it would depend on how sloppy it is. How much end play does the driveshaft have? (when you butt it all the way into the trans how much distance is there to move it back till the rear ujoint is seated). excessive endplay is more likely to be a cause than a worn bushing (unless it is real bad) & right now I'm thinking the bushing ain't the problem (it might not be perfect but I am thinking elsewhere) however too much end play can definitely be it. I like ~3/4-7/8, enough to get the ujoint over the yoke when installing plus a bit more measured with springs compressed at ride height/weight (stands under axle). plus we have the angles and the pig as potentials. As Cab says we gotta stay after it.

the front to back distance would be measured with the rear ujoint loose just like you were first installing the driveshaft (& that is a good time to check that) when you butt it all the way forward to clear the rear yoke then move it back to seat into the yoke, that distance. Right now the side to side slop the front yoke has in the extension housing bushing you mentioned, 1/4 seems like a ton or did I misread something.

Did you put poly mounts in the trans yoke as well?

Well fellas the only different thing is the TRANS, so I'm thinking the tail shaft bushing or the bearing. I can't remember if the 904 and four speed use the same length shaft.

Almost immediately after I left the diff shop for the gear change, I felt the resonance on the highway above 60mph.

that's what Sherlock Holmes/Columbo and lieutenant Kenda would (all) call a major clue. Keep It simple Sam. Unless they messed up the shaft somehow during the swap (ujoint clips/caps can be delicate/problematic) I'd be thinking about swapping the pig at least for a test even with an open gearset. I know it is a pain.

I missed that it ain't an 8&3/4. do keep us updated.

There should not be any movement with the driveshaft installed in the car , no up or down and no side to side movement, EVER It should rotate smoothly in a straight plane, no wobbling
Wobbling(up or down or side to side) makes vibrations, straight and true do not

I wonder of I am seeing it right. It appears that the yokes are not in alignment. They have to be within a few degrees of each other to avoid vibration. The u-joint connection speeds up and slows down at every rotation. I'd expect trouble if the u-joints didn't both do this at the same time every rotation.

the first thing I would do right now is pull the rear ujoint caps & move the shaft all the way forward & measure the distance back to where the caps seat in the yoke.

If the car is on the ground in the last set of pictures the driveshaft is about an 1" short.

Can you verify both U joints are in line on the driveshaft? The driveshaft looks to me to be too short...If it's not into the tail shaft bushing far enough it does wobble and will cause a vibration. The splines and the tail shaft do not control the yoke as much as the bushing in the tail shaft housing does.

I will check that out. About 1.75" exposed of snout and no less than 5/8 of snout is engaged in trans.

Where the rear pinion bearings changed? Maybe that front pinion bearing is bad, nut tight? That might be the place to check since the "noise" came after the gear change. I thought I read you replaced it all at the same time.
Also since you had to change front slip yoke, is it bent? Have to watch it turn running in gear. Only way to check the driveshaft for harmonics is to try another one.

Do you have a angle finder, compass?
If so measure the pinion angle and driveshaft angle and let us know what you find yours are, if you don't have one or can't borrow one go get one (beg, borrow or steal one ) and check your pinion and driveshaft angles with the car on the suspension like it driven

Came in here for this. Have you checked your pinion angle? You can exacerbate other driveline vibrations if your pinion angle is bad.

The car is on the ground. Yes, the driveshaft could be a tad short, but I am not grasping why it's now an issue when it wasn't prior to the gear swap.

I am not sure what you are referring to. You mean the trans mount? if so, it is just a regular mount, not a poly.

Measure the inside spline length
I like to see from .750 to 1.0 yoke extension out of the tranny tailshaft housing from bottomed out with the car on the ground loaded ready to drive You have way more than that on yours
The one picture of the front tranny yoke on the driveshaft, it looks like to me that the drive shaft is not centered in relation to the yoke center I'm sure it is not out of center due to if it was as bad as I see it you wouldn't be able to drive it over 20 MPH without the drive shaft breaking the output shaft and tail shaft housing on the tranny

Just by chance when they were installing the u-joint maybe a couple needle bearings got buggered up? Just thinking out loud.

One other thing to consider is what RPM the drive shaft was balanced at.

You need a high speed balance.

and most shops that do general driveshaft work for everything, [cars,trucks,big rigs] do NOT do high speed balancing. it seems that even though it costs a few dollars more, one might find a necessity to use a race type driveshaft company to insure the shaft is not only high speed balanced, but also to insure it has almost zero runout.

eyeballing your driveline angles is not valid

Measure them

come back when you have numbers

First, get a driveshaft that's 3/4-1" longer.

Second, you said less of a vibration and more of a hum... any chance that you're getting exhaust resonance with the higher engine RPM while cruising?

I am beginning to look at having a 3”, .0083” wall thickness, driveshaft built, that is .750” longer on the body to get more engagement in the trans.

I would butt the yoke you plan to use all the way into the trans then measure the C to C you have at that point then subtract 3/4 from that dimention & this is with the suspension loaded (axle on stands). give that number to your shop.

As log as your keeping the same size U joints your good on the straps and bolts
Do NOT over tighten them

Uppn more measuring, the body of the driveshaft definitely needs to grow 3/4" to be right. Should have a new DS tomorrow.

Are you going by rear ujoint endplay (I hope so) or by front yoke appearance/it's end play?

Pushing the shaft all of the way into the trans then backing out .750”-1”

I would suggest .750" max (springs loaded)

And have we checked pinion angles yet?

It sucks when you think you have a problem pinpointed and spend money only to see that the symptoms continue. ALL car guys deal with this!

I have a friend whose Mopar mechanical and automotive skills I have great respect for, insists that a driveshaft yoke shoved into a transmission bushing with too much clearance is the root cause of many unexplained intransigenet driveline vibration issues.

He recommends a new bushing which is too tight and honing it out to fit the specific yoke you are using. He suggests with the transmission out of gear to relieve stress on driveline, to attempt to move the yoke up and down. Any permitted movement is indication of potential problem.

I will forward this thread to him for comment.

I have a friend whose Mopar mechanical and automotive skills I have great respect for, insists that a driveshaft yoke shoved into a transmission bushing with too much clearance is the root cause of many unexplained intransigenet driveline vibration issues.

He recommends a new bushing which is too tight and honing it out to fit the specific yoke you are using. He suggests with the transmission out of gear to relieve stress on driveline, to attempt to move the yoke up and down. Any permitted movement is indication of potential problem.

I will forward this thread to him for comment.

As my friend is not a member of this forum but he did send this response to me after I had forwarded a copy of this thread to him. The following is a "copy and paste" of his response.

Roger:

It looks like you explained it well but I'm happy to provide you something you can cut and paste as I am not a member there. However some members on this site may carry over to the FBBO forum and would recognize me by my handle Meep-Meep.



The sign of slip yoke vibration is to drive the car close to or at freeway speed and find that spot where you "float it" by not being under power or deceleration. Just modulate the throttle until you hear that grumble sound and you will find you are not loading the drive line, thereby allowing the yoke to rattle around in the tail housing. This is independent from a balance issue since an out of balance shaft will try to orbit around it's center of mass even with a properly fitted bushing.

It's been my experience lately with slip yokes that the clearance is way too loose even when replacing the parts with new. And by way too loose I mean over about .007" is too loose. For a quick test on an unloaded drive line (tranny in neutral and rear wheels off the ground) grab the front yoke and attempt to move it up and down and you should have little to no perceptible motion. I don't have the actual dimensions off the top of my head but for sure the new yokes that I measured are small and the bushings are perhaps bigger than they should be. My yoke reference is taking the measurement on the non worn part on a factory yoke. One car I had measured over .060" and others are in the .020" range if memory serves and all have the grumble to some varying degree. Granted taking a dial indicator measurement way out at the U joint will show a pretty big number but it's still a reference.

The fix is to fit the bushing to the yoke with the proper clearance, and since the yoke is the part more likely to be replaced or changed, it would make sense to make an undersized custom bushing and hone it to fit the new yoke. For honing I will take my next tail shaft and yoke to a machine shop that has the Sunnen hone like the kind used to resize rods and tell them it's like fitting a piston pin (but not piston pin clearance). This will ensure a non tapered round bore. The only catch is the bushing bore needs to be in proper alignment with the output shaft bearing all the way up to the pilot bearing or front pump bushing.



Alex Lobodovsky

This months issue of Hot Rod (April 2017) has an article helping a guy with the same problem your having,only his was worse. Hot Rod Rescue is the article. He went through everything, even taking it to a 4x4 shop because he was being told his angles were off. It was a combination of a lot of things, including transmission dowel pins out of alignment. Right now your just speculating and throwing money at a problem hoping to find a cure. Pick it up and read, very enlightening.

Progress report!!!

FWIW my buddy had a vibration like you describe in his 66 fairlane with 4 speed, pulled apart the trans and had 2 bad bearings along with a bad trans mount that made it worse.

Just wondering if you got to the bottom of this. I just had my Goodyear polyglas tires road force balanced and got some improvement. The increased rpm may bring out a tire balance issue that you didn't have previously. Let us know what you have found.