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RT Diff Mount with Energy Suspension top mount and L28


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So I am installing an RT Diff Mount with the Energy Suspension PU top mount (GM style) bushing. I am also keeping the L28 drive train.

Has anyone done this mod and corrected for the lower drive train angle that occurs with the RT mount? Have you shimmed the mustache bar in order to return the driveshaft to diff angle back to factory spec? I am using the AZC mustache bar and was thinking of reusing the factory mustache bar washers (minus the rubber) to shim the bar down approx 1/4" to get the rear of the diff lower and correct the angle. I know the RT diff mount was originally designed to accommodate the V8 swaps, so the drive line angle is a concern for the original L28.



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Is the 3.1158 the same internal structure? Does it have the "catch" built in to hold the bushing together in case of failure?

From TechnoVersions site where I got this one:

My concern is the 2 degrees of difference that seems to give some people the vibration.

Optional Top-Mount:

An alternate way to mount the differential is with a modified Energy Suspension urethane top-mount. This allows the differential to be mounted completely from the top, eliminating the need for the original stock bottom mount. This is a standard Energy Suspension part number 3-1108 that has been modified to have its hole-spacing slotted to fit the mounting bolts on a Z-car differential. In addition to slotting the mounting points, the Energy Suspension part also has been machined in the urethane area between the mounting tabs to allow better fitment. This mount is much stronger than the stock Z-car mount, being less pliable, and having an internal structure that prevents excessive movement. It is also under compression, rather than tension, when power is applied.

The Energy Suspension top-mount places the front of the differential slightly lower than when stock, and the original bottom mount must be removed. This change in geometry (about two degrees of driveshaft angle) is not significant in stock applications, but leads to slightly better driveshaft angularity with typical V8 applications.

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According to TechnoVersions:

7 minutes ago, wheee! said:

is not significant in stock applications, but leads to slightly better driveshaft angularity with typical V8 applications.

If you want to be really precise about it, I'd assemble it like it is and measure the angles, then decide about shimming the mustache bar or changing the bushing

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Edit - forgot to say that people have been known to grind out a hollow for the nose of the diff to get a proper fit.  That will raise it some.  Enlarge the side holes a bit, do some grinding, a couple of shims on the bar...

Just to fill in, I made a picture.  5/16" shorter. 

They also have one that says V8 only.  But it's cheaper.  Can't really figure out the page - http://www.energysuspensionparts.com/search.asp?pg=1&stext=3-1108



5 16 ES mounts.PNG

Edited by NewZed
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This just in! I carved out the poly bushing with a wire wheel on the grinder. Then I mounted the RT mount in place as well as the lower support. 

Measuring the length of the diff bolt through holes at 3", I placed the old (filthy) lower mount in the lower support and measured the clearance from the mating surface to the poly bushing mating points which are flush on the top of the diff nose. 

The height is exactly 3"! That means that the diff will be in the exact same position now when I mount it with the poly bushing and no lower bushing. The lower support will remain for strength. Lots of clearance between the diff and the lower support. No further adjustment needed. 







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I would have thought the main part of the weight is held by the mustache bar and that the nose would be fairly stable with the poly mount.... How much did it droop? The poly bushing has the safety catch in it so the bushing can't fail and let go, but I expected the nose of the diff to be held fairly solid after scalloping out the PU so that the mounting tabs were flush with the diff. Do you have a pic of the lower pyramid bushing you used?

Edited by wheee!
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I think your right Wheee, cut out like that will allow the tabs to lay flat, not sure what he is referring too. There's is no rea weight on the nose of the diff .

i did mine like yours as a sandwich . I still get a thud occasionally so I must have something else going on.

I also ran my diff just hanging from the RT mount without issue

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Edit: wait I had that wrong.

The weight is supported a bit by the rear, but if you just hang the diff in there you will see the front of the diff nose down as the bar twists. The bushing at the top can hold it up, but the bushing will kind of fold in, at least on mine it did a bit since you are removing the material via the slots and the PU between the legs, it has an internal structure to keep it from fully separating even if the PU is gone, but it is there to keep the bushing in one piece not to prevent flex. I put an adjustable height bump stop bushing underneath to support the nose of the diff so it sandwhiches the diff without it being held top and bottom. Makes getting the rear arms off easier by a little bit.


Edited by seattlejester
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Ok. So the PU bushing has been carved away to be the same shape as the top of the diff nose. The tabs will bolt flush to the diff and the PU will be sandwiched between the diff nose and the top of the RT mount. Shouldn't flex inwards at all...

I may still add a small support bushing between the lower support and the bottom of the diff nose just because I can. The rear moustache bar is the Arizona Z Car billet aluminum piece so I don't see much flex with that in place along with PU bushings. 


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That is one beautiful-looking undercarriage!  

FYI -- a lot of folks don't use the lower diff mount after installing a RT-style upper mount.  You obviously still can use it, if you want, but may not be necessary depending on your intended usage, suspension setup, engine output, etc, etc.

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