9.25 Axel issue. Need input Please

[WissaMan]

I own this car now and want to get this vibration problem sorted out. When looking at the differential in relation to the driveshaft it looked like to was tipped down noticeably, so I went ahead and ordered the Miller tool that is mentioned in the FSM (found one on ebay!). The angle at the diff is +7º which is pretty far off (spec is +3). So I'm pretty sure this is the source of the vibration. The angle at the tailshaft is +1º -- right on spec.

I know about getting shims and all, but before I do that, I'm wondering just what could be causing this. Ride height is normal. It does have air shocks but pumping them way up vs. flat didn't change the measured pinion angle. Could the new springs be made wrong? Even just looking at the springs it looks like they really cock the axle forward, but maybe that's just how Formals are?

 

[Cop car wagon]

Iso mount leaf springs have a reverse arc in the front section. To the untrained eye, it makes the spring look sagged.
A standard spring is a C shape, and iso mount uses an S. Of course, we are talking to a minor degree.
Stack some tapered shims in there.
Just reading this thread for the first time makes me wince with the missteps.

 

[WissaMan]

Seems like 4º shims should get it to the +3º spec. I've been looking for shims specifically for this rear-end but I guess they are somewhat universal? But I think I'd want them with the cutout rather than the hole for easier installation. What do you think about these? And why do so many of the shims come with the nuts and bolts?

Belltech 4977 Belltech Pinion Shims | Summit Racing

 

[Cop car wagon]

4° should.
Now you have the isomount still in place. It uses a large ( approx 1 1/4") hole in the diff housing to center, with a stand wall that fits in from the upper rubber support.
You will need to buy a solid shim plate and enlarge the center hole to fit over the locater wall.
You might even need to add height to the stand wall on the plate, as its very short, and the shim will use up the height.
The old mopar performance,( Direct Connection) sold shims with the large hole.

 

[CBODY67]

All rear axle housings have some sort of locating device on the bottom of the housing for the main leaf of the spring to index with for positive location of the spring to the axle. Be it a recess or something different. Not just the u-bolts which attach everything to the spring perch on the axle housing ends.

Similarly, the spring pack has a center bolt that keeps the leaves located as they need to. The pictured bolts look like what I recall being that center bolt. I suspect the slot in the shims would be to clear that center bolt and its nut on the bottom, so the shim slides into a loosened retainer situation rather trhan having to remove the lower clamp of the spring to install the shim.

I'm suspecting there is a Chrysler MasterTech video on driveline-related vibrations and how to measure and correct driveline angles. Might have to search through them at www.mymopar.com, but they would be more universal in nature rather than dealing specifically the rear axle in your vehicle and model year. Even more universal would be the related YouTube videos.

Just some thoughts,
CBODY67

 

[WissaMan]

The FSM says the shim must be no more than 1/4" thick measured at the center of the shim, so I assume that is so the locator pin/stand wall can still peek out above it. I definitely don't want to have to put extensions on them as that would probably require dropping the axle completely.

I just found a 4 degree shim with measurements and it shows .485" thick at one end, .135" thick at the other. So by my calculations it should be .175" thick in the center, so I think a 4 degree shim will be ok.

 

[Cop car wagon]

Worst case scenario is having the car and rear end housing supported, and having to remove u bolts, and possibly the iso mount plates and bottom of shock. Everything (driveshaft, brakes lines) can stay connected. Do one side at a time with the other side loosened.
The iso mount plate stand wall is not tall. I'd show you a picture, but I have converted all my 73 up cars and thrown that stuff away.
The spring center bolt of old is now surrounded by rubber, and no longer plugs into the housing. Hence the need for a shim with a big center hole.
( I will post up a picture of that)

 

[WissaMan]

I'll order up a set that just has a center hole and plan on boring them out with fat step bit.

 

[WissaMan]

Is that picture of the shim from your archive or is that something you have handy that you could measure the hole?

 

[Cop car wagon]

I can measure. I have a vast collection of everything mopar. And I do mean enormous, vast, overwhelming

 

[WissaMan]

You sir are a fine gentleman!

 

[Cop car wagon]

1.700"
I no longer have a iso rear end to measure the locating hole to know if that is an exact fit or clearance fit.

 

[WissaMan]

Thanks!

 

[WissaMan]

This is fixed! But it took way more work than I would've liked. I bought a set of 4 degree shims with the plan of drilling out the center so they'd fit the much larger centering holes of the iso mounts. But as it turned out, they were too wide and the centerline thickness of them was more than the 1/4" that the manual said they should be and I was pretty sure they were not going to allow the centering flange on the iso mount to protrude up through them into the hole on the axle mount.

Wanting to get it done this weekend I cut them up and fab'd my own shims using just a part of them. When I got it all back together I measured the pinion angle and it was +2.5. I took it for a drive, to my relief (after all the effort) the nasty vibration is gone and the car is now smooth as glass at all speeds. I'd still like to have actual correct shims that just fit, but I couldn't find any online. I'm thinking of 3d printing some out of a tough carbon fiber reinforced Nylon.

I can't explain why it was off so much other than the new springs were made incorrectly. That's the only explanation I can think.

 

[Cop car wagon]

I agree, the front segment of the spring had too much arc.
I know the thrill of having something working the way it should, after fighting it. Congratulations!
You can now rule the roads in a C body.